CA2114030A1 - Infrared-based sensing circuit providing an output simulating the output of a flame rod sensor - Google Patents

Abstract

A converter circuit (11) provides an output signal of polarity opposite that of an input signal having a DC component and a low frequency periodic component in response to presence of that input signal. Current is transferred from a first capacitor (32) to a second capacitor (50) to accomplish this conversion. The circuit has particular application in a flame sensing interface circuit using a sensor of the infrared radiation generated by the flame to change the impedance of the sensor. By use of the circuit, the output of an infrared sensing amplifier imitates the output of a conventional flame rod sensor, and provides an output signal compatible with the output of a flame rod to the input to a flame signal processor.

Description

'`'O 93/12387 21 1 4 ~ 3 û PCI/US92/11111 D~FRARED-BASED SENSING CIRCUIT PROVIDING AN
OUTPllT SIMULATING THE OUTPUT OF A FLAME ROD SENSOR
, BACKGROUND OF THE INVENTION ,`
State of the art controllers for fuel burners such as furnaces are now based on microprocessors which dramatically improve the control process. ,~;
Nevertheless, it is still necessary to provide information as to the current operating state of the fuel burner. Among the most important of the state parameters is whether there is flame actual1y consuming the fuel being provided to the burner. The continued supply of fuel to the burner must be conditioned on the presence of flame, since if flame is not present and fuel is allowed to flow to the burner, the accumulation resulting can explode or asphyxiate, either one a potentially lethal event.
Accordingly, it has been recognized for a long t~ne in burner control technology that detection of flame is of paramount importance.
There are basically three kinds of flame detector elements. Perhaps the simplest is the so-called flame rod, which is simply a metal element insulated from ~, the burner metal and located within the ionized particles forming the flame, andwhich forms with the burner metal a sort of diode element when flame is present.The diode action arises from the difference in the size of the flame rod compared to ~;
the burner itself. An AC potential applied between the flame rod and the burner metal causes DC current to be carried by the ionized particles generated by presence -~
of a flame. By detecting presence of this DC current flow, it is possible to determine presence of flarne. Because of the difference in sizes of the flame rod and the burner, the current flow is from the flame rod to the burner, meaning that presence of flame is signified by current flow into the flame rod signal conductor, placing its potential below ground voltage as represented by the burner.
A second type of flame detector circuit uses a phototube sensitive to ultraviolet radiation, and which produces a characteristic change in impedance indicating flame when such radiation is present. United States patent application Ser.
No. xx, filed on December 6, 1991 by Scott Peterson, entitled "Fail-Safe UV
Amplifier that is Compatible with Flame Current Digitizer Circuiti', and having a common assignee with this application, uses the special characteristic of the ultraviolet~ensitive phototube to discriminate between actual presence of ultraviole radiation a~d other causes of change in phototube impedance. The circuit of thisapplication is noteworthy for the reason that it produces an output which simula~es lhe output signal of the conventional flame rod sensor.
A third type, and the one with which the invention to be described deals, produces an output when infrared radiation produced by a flame impinges on WOg3/12387 21140~ 2- PCr/USs2/ll~l an cadmium sulfide or other type of detector tube whose impedance drops in response -to the radiation. Each of these sensors produces an output requiring substantialprocessing by special circuitry before a signal indicating presence and absence of flame and which is suitable to be an input to a microprocessor is generated. The5 circuitry which converts the flame detector signal to a signal suitable for use by the controller is referred to as a flame amplifier and its output as a flame present signal, or more si nply, a flame signal. In the case of infrared sensing, the characteristic of the signal which reliably indicates flame is known to be the presence of a 5 to 15 hz.
flicker. This flicker is a natural result of combustion of the hydrocarbon fuels in 10 common use, and many infrared sensor circuits use this flicker as a basis forindicating presence of flame. The flicker is only one component of the total infrared radiation output of the flame however, and a minor one at that, so those infrared sensor circuits relying on the flame flicker to distinguish between presence andabsence of flame must carefully separate the flicker component of the infrared sensor 15 from what is noise for the purposes of flame detection.
A flame rod amplifier circuit designed to operate with a positive DC ~ -power supply adds a measure of reliability to its operation by interfacing with a flame - rod sensor whose output is a negative current, i.e., one whose current flows into the sensor from the flame amplifler. The extra measure of reliability arises from the fact that any 1eakage current within the flame amplifier cannot masquerade as the negative current flow fonning the flame rod output. Any leakage current in a flame amplifier powered by positive voltage will almost invariably be positive, and thus not likely to be interpreted as the negative flame rod sensor output. A pending US patent application which covers a flame amplifier circuit embodying these concepts is titled Fail-Safe Condition Sensing Circuit, was filed on October 28, 1991 with Ser. No.07/783,950, and has a common inventor and assignee with this application.
The most efficient way to implement this flame rod amplifier is as a special purpose microcircuit. Because of this implementation, returns to scale are particularly high, meaning that the unit cost drops substantially with increases in the number of individual circuits produced. Accordingiy, it is very advantageous for this flame rod amplifier to be compatible with not only the flame rod detector, but also with the UV and IR detectors. However, the power required to drive the UV and IRdetectors is;different from that required for flame rod detectors, and the output signal of each has substantially different characteristics from the other two. Accordingly, it is not possible to simply replace the flarne rod detector with either a UV tube or infrared cell flame detector. Instead, the interface circuit described above for the UV . -tube, o~ the interface circuit of which the invention to be described forms a part, is 211~03~

necessary to provide a signal output compatible with a detector intended for usewith a tlame rod detector.
~-There are any number of circuits now known which are intended to detect presence of flame by sensing presence of the 5-15 hz. frequency components in the infrared spectrum. For example, recently issued U.S. patent no. S,073,7~9 `
discloses apparatus for detecting signal components indicative of flame by use of the discrete Fourier transform. ,-~
There are also relevant circuits which filter rectified signals for various purposes such as regulating the gain of detection circuits by controlling the amplitude of the voltage provided to the input stage of the detection circuit asnegative feedbaclc. The chief example of such a circuit as now known is found inU.S. Patent No. 3,137,822 issued June 16, 1964 to Anderson.
,"~.
BRIEF DESCRIPIIO~ OF THE INVE~TION
The embodiment of the invention to be described has the ability to interface the above-described flame rod amplifier to the standard infrared flamedetector cell. This interface circuit depends on the presence of the flicker component of the infrared radiation to provide a flame detector signal indicatin~
that flame is present. The signal directly produced by the infrared detector cell is processed by conventional circuitry including active filters to finally provide a -~
signal approximating a sine wave when flame is actually present. The part of this - interface circuit which forms the subject of this application receives this approximate sine wave signal and uses it as the basis for generating a signal nearly identical to the signal provided by the flame rod detector when flame is present.

2 5 To operate compatibly with the amplifier or processor for a aamerod detector, it is necessary for the i~frared detector circuit to produce a signal which simulates the flame rod output. At the same time for absolute safety in operation, it is necessary for the circuit to operate on power whose polarity isopposite the polarity of the simulated aame rod signal which the circuit must 3 o provide. A circuit which provides such an output current signal of a tïrst polaritv is generally intended to operate with a low frequency input signal varying between ~irst and second voltage limit values each of a second polarity different from the first polarity. In the context of infrared sensor-based flame detection here, the input signal has an approximate sine wave shape. The circuit is powered by a DC
3 5 source whose voltage is of the same polarity às the voltage limit values and is thus of polarity different from that required for the output signal.
This invention has a half wave and a full wave embodiment. The half wave embodiment is much less likely to be used in an actual application but is included as an S l3BSTlTlJT ~E~:~

. 211103U
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aid to understanding the invention and for completeness. Both embodiments of -~
this invention comprise a converter -circuit providing an output signal of a first polarity responsive to a periodic lo~
frequencr input signal varying between two voltage limit values of a second s polarity different from the first polarity and SUY3STITUTII~ T

211~0 ~0 ~`:

having at least a predetermined amplitude. The input signal thus has a substantial DC component on which the periodic component is superim~osed.
A first part of the half wave embodiment of the converter circuit includes a first capacitor whose first of two terminals is connected to receive the s input signal. A first resistor connects a second of the first capacitor's two terrrunals j~
to ground. A diode circuit comprising a diode and a second resistor in series connection has a first o~ its two terrninals connected to the second terminal of the first capacilor with the diode in the diode circuit oriented for back biasing by the input signal. A second capacitor is connected between ground and a second terminal of the diode circuit and provides the output signal at the second terminal of the diode circuit. A resistive path is connected to discharge the second `
capacitor over a period of several input signal cycle times, and may comprise eilher a third resistor connected across the second capacitor, or a third resistor in parallel . . .
with the diode.
As the input signal voltage varies between the two voltage limit values, the first capacitor quickly charges to near the voltage of the input signal's DC component. The swings of the input signal which cause the voltage at the cornmon connection point for the diode circuit, the first capacitor and ~he first resistor to cross 0 v. then forward bias the diode, and the second capacits~r charges through the resistor of the diode circuit to a polarity different from that of the input signal. It can thus be seen that this circuit makes it possible to produc~ ~n output signal which indicates presence of the input si~nal with a volta~Je on the second capacitor of polaritv different from the polarity of the input siYn~l. The resistive path allows the second capacitor to quiclcly discharge whenever the periodic component of the input signal disappears. Thus, any current le~kage within the circuitry involved with producing the input signal, to the circuit otinterest here, will not simulate the input signal and cwse presence of the output signal of opposite polarity. Accordingly, this circuit is suitable for applic~tions ;~
where high reliability is important, such as the aforementioned flame si~,nal ~-3 0 processing.
In certain safety-critical applications, it is further necessary to reliably detect large changes in the input signal. To accomplish this, a secon~i part of this converter is provided which includes a level shift detector sensing the input signal voltage and providing an output signal voltage having the first polaritv responsive 3 5 to a steady state input signal voltage and the second polarity otherwise. A diode circuit comprising a second diode and a third resistor in series connection conducts the output signal from the level shift detector to the second resistor's second terminal with said second diode in said diode circuit oriented to be back biased by an output SJ~aTl~l~TE ~3~E~

21~Dv~J~ ;'i'.

-4.1-signal voltage of the first polarity ~rom the level shift detector. The leve! shift detector output obliterates the output &om the first part of the converter circuit, whenever a large change in the input signal voltage occurs. If the input voltage ~.Zj'~
were to be supplied by a flame detector cell whose impedance increases ' dramatically when infrared radiation falling on it decreases due to loss of the ~ame, then the second part of this converter circuit acts to signal this condition. ~, The converter circuit of the preferred embodiment operates in a full wave configuration in a manner very sirnilar to the simplified half wave circuit configuration described above. As with the simplifiec~ circuit, the full wave ' converter circuit also provides an output signal of a first polaritv responsive to a -periodic low fréquency input signal varying between two voltage limit values of a second polarit,v different from the first polarity. The second part of the converter circuit can also be employed in this embodiment to detect large swings in the input , signal voltage. This full wave converter circuit comprises a first capacitor having a `~
~lrst of two terminals connected to receive the input signal. A~ inverting amplifier -also receives the input signal and providing a signal varying between the two voltage limit values, and inverted with respect to the input signal. By inverted in this context is meant that the inverted signal's voltage at any instant equals twice 2 o the voltage of the ,,-. . .

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~O 93/12387 2 1 1 4 0 2 f pcr/us92/11111 DC component in the input signal less the value of the DC voltage component at that instant. More simply, the inverted signal mirrors the input signal about the DC
compc,nent voltage level. Both the input signal and the inverted signal have identical DC components. A second capacitor has a ~Irst of its two terminals connected to receive the inverting amplifier output. First and second resistors connect second terminals of the first and second capacitors respectively to ground. First and second `-diodes have their first terminals connected together and their second terminals connected respectively to the second terminals of the first and second capacitors.
These diodes are oriented so that they are back biased by the input signal and the inverted signal. A third resistor has the first of its two terminals connected to the first and second diodes' first terminals. A third capacitor is connected between ground and a second terrninal of the third resistor and provides the current signal at the third resistor's second terminal. A resistive path is cormected to discharge the thirdcapacitor.
As the input signal varies between the two voltage lirnit values, the first and second capacitors both charge to the DC component voltage. As the voltages at `
their second terminals are driven below 0 v. on succeeding half cycles, the diode - connected to receive the signal is placed in conduction, and part of the charge on the fisst or second capacitor is transferred to the third capacitor, but with the opposite polarity. In this way the third capacitor is charged to a polarity opposite that of the voltage limit value when the low frequency periodic input signal with a DC
component is present. Should the input signal not vary within the specified range~ the `
charge on the third capacitor drops substantially. This circuit is also suitable for critical safety applications such as indicating that flame is or is not present. Because of the full wave characteristics by which the third capacitor is charged, the indication is made with greater precision and reliability.
Accordingly, one object of the invention is to provide a signal of one polarity when a periodic signal varying between voltage limits of the opposite polarit,v is present.
A second object of the invention is to provide a means of confonning the signal provided by a flame detector sensitive to infrared radiation, to a flame rod signal.
Another object of the invention is so far as is now known to be possible, to immunize the circuit which processes a signal, from leakage or other malfunctions which might simulate its presence.
Other objects and purposes of the invention will become apparent from the descriptive matter following.

WO 93/123X7 PCr/US92/1~11 BRIEF DESCRIPTION OF THE DRAWINGS ~;-Figs. l and 2 are circuit diagrams of two similar simplified embodiments of the invention.
Fig. 3 shows waveforms helpful in understanding the operation of the - 5 circuits shown in Figs. l and 2. `~
Fig. 4 is a circuit diagram of a preferred embodiment of the invention, -part of which is shown in block form, with individual components shown for the part of the circuit involving the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT ;
The converter circuit of Fig. 1 (and the very similar Fig. 2) is a simplified embodiment of the invention and not now considered to be the preferred -embodiment. It is included for the purpose of disclosing the structure and operating '~
principles of the invention. Since this circuit functions in a half wave mode, its - -resolution is lower than the operational circuit of Fig. 4 and there is a more pronounced ripple in the output signal. However, in certain applications, this circuit ,;
will function adequately.
Waveforms helpful in understanding the circuits of Figs. l and 2 are shown in Fig. 3. The letters designating individual waveforms display the signal at ~ -the similarly designated signal paths in Figs. 1 and 2. In Figs. l and 2, a sine wave input signal shown as waveform a in Pig. 3 is applied to input terminals 29 and 30.
The input signal applied to is typical of that provided to the preferred circuit and has a DC component of around +4.35 v. and peak excursions falling between positive , -~
voltage limit values of approximately 2 v. and 6.5 v. With the voltage of the input signal thus always positive, it can considered to be of a single polarity. Inputterminal 30 forms the ground for the circuit, with respect to which all voltages are referenced.
A capacitor 32 is connected by a first of its two terminals to input terminal 29. The second terminal of capacitor 32 is connected at point b in Figs. 1 and 2 to input terminal 29 (ground) through a charging resistor 40. The second terminal of capacitor 32 is also connected to a first ter ninal of a diode circuit comprising a signal diode 45 in series with a signal resistor 48. Diode 45 is oriented with its cathode connected to the second terminal of capacitor 32 so that the positive ;
polarity of the input signal back biases the diode. A second terminal of the diode circuit is connected to a first terminal of an output signal capacitor 50, whose second terminal is connected to ground.
The capacitor 32 quickly charges to the DC component voltage level through resistor 40 with point b relatively negati~ e with respect to point a. During ~lO 93/12387 2114 0 ~ ~ pcr~us92/~

the relatively positive half cycles of the input signal, point b will still be positive as is ~ -shown in waveform b of Fig. 3. However, after this charging is complete, at around .2 sec. as shown in waveforrns a and b, then during the relatively negative half cycles of the input signal, point b is driven by the voltage across capacitor 32 to approximately -2.0 v. The level of -2.0 v. arises mainly from the displacement of the ~ -~
input signal voltage below the DC component voltage level. This negative voltage at point b forward biases diode 45, allowing current to flow through the diode circuit ) ~ ~
from capacitor 50 and into capacitor 32, in effect transferring charge from capacitor -32 to capacitor 50. The charge transferred, however, negatively charges capacitor 50 and causes point c to become negadve with respect to ground. This negative charge on capacitor 50 provides the output signal of opposite polarity which results from the presence of the specified input signal. As the amplitude of the low frequency component in the input signal drops in the interval from 0.7 sec. to l.0 sec. as is shown in waveform a, the voltage across capacitor 50 becomes correspondingly smaller, as is shown by the rise in voltage of waveforrn c after 0.7 sec. This i -reduction in the voltage of waveform c arises because there is constant discharge of ' capacitor 50 through resistor 56 and load Sl on the one hand, and no charge transfer 7"'"
from capacitor 32 to capacitor 50 on the other, this latter because of the reduced peak .
to peak voltage beginning at 0.7 sec. for the waveforms shown. That is, when theamplitude of the input signal's periodic component nears 0 v., the voltage across ~
capacitor 50 also nears 0 v. because of discharge through the resistive path comprisin~ -in Fig. 1 resistor 56 and a load 51. ~ ;~
The circuit of Fig. 1 may be configured to provide either a current or a voltage output signal. If the irnpedance of load Sl is very high compared to the value of resistor 56, then the level of the voltage across load 51 provides the output signal.
If load Sl has very low impedance, then the output signal is provided by the curren level through load Sl.
In Fig. 2, the resistive path for discharging capacitor 50 is formed by resistors 40 and 48 and by resistor 34 connected across diode 45. In this embodiment, the output signal is considered to be the voltage across capacitor 50. ln both Figs. 1 and 2, the voltage at point c is below ground and the voltage signal is negative when indicating presence of the input signal. In Fig. 1, when load 5l is of ,;
low impe~lance and the current level defines the signal, current flow into point c repreænts a polari~,r of the signal in waveform c different from that of the input signal. It can thus be seen that the circuit of Figs. 1 and 2 is admirably suited for processing a flame signal represented by the periodic component of the input signal without providing an output signal falsely indicating presence of the periodic component.

Wo 93/12387 ~,~ ~ ~a PCr/US92/~
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The preferred circuit of Fig. 4 is shown in the context of the complete ' ~ -circuit for processing a signal formed by the infrared radiation from a conventional , ;
burner flame. An infrared detector cell 10 is placed in proximity to the flame to be detected. In these types of cells, resiseance is inversely related to the level of impinging infrared illumination. In one type of these cells, dark resistance is approximately 1.2 MS2 and falls to .6 MQ when intense infrared radiation impinges on the cell. The response of a typical cell 10 to changes in the intensity of impinging infrared radiation is at least in ehe microsecond range, so ehe dynamic resistance changes accurately reflect the actual changes in infrared radiation intensity.
In the circuit of Fig. 4, a power supply provides output voltages of 8.7 ~-;
v. and 16 v. on conductors 15 and 16 respectively. Resiseors 13 and 17 form a voltage divider circuit with infrared cell 10 between the 16 v. output of power supply 12 and ground. As the resistance of cell 10 varies, the voltage at point 20 also varies, providing the infrared cell signal to the circuit. An overvoltage protection circuit 18 prevents static discharges from damaging the inputs of delicate amplifiers in the signal processing elements. l -If for some reason flame vanishes completely over a very short period of time, the resistance of cell 10 will rise rapidly, and the voltage at point 20 will also rise rapidly. Level shift detector 19 senses any rapid rise in the voltage at point 20 ! i and provides a relatively high positive voltage on its output path 21.
More normally, the voltage signal at point 20 will be produced both ' by changes in the impedance of cell lO arising from the changes in the level of I `
infrared radiation falling on the cell, and by other effects, all of which combine l-during and for a period of time after flame has ceased within a furnace, to form a composite signal having a nwnber of different frequencies. It is known that there will be a characterisdc frequency in the range of 5 to 15 hz. present only when there is an actual flame existing. By detecting this characteristic frequency, presence of flame can be accurately determined.
In order to extract this characteristic frequency, the voltage signal at point 20 is procesæd by a æries of signal filters 23, 24, and 25, filters 23 and 25 being active filters of the filter^amplifier type and which receive a standard voltage -provided by a precision vo1tage halver 22. Filter-amplifiers 23 and 25 have active elements~which are powered by the voltages available from power supply 12 and whose poiarities are both positive. This processing is conventional and results in a signal on path 29 which closely conforms to waveform a of Fig. 3. The internal output amplifier 26 shown as a part of filter 25 receives the precision voltage from the voltage halver 22 through a resistor 31 on path 27. By providing the precision voltage to one input terminal of the output arnplifier stage of ~llter 26, the DC

~o 93/12387 ' 2 1 1 4 0 3 ~ PCI/US92/lllll component necessary for proper operation of the converter circuit 11 can be easily created. , Within converter circuit 11 an operational amplifier 36 inverts the input signal. As explained earlier, the term "invert" is here taken to mean that the voltage '~
of the inverted signal is equal at any instant to twice the DC component of the input ;',' signal, 2 x 4.35 v. or 8.7 v., Iess the voltage of the'input signal at that instant.
Amplifier 36 receives the input signal on its - input terminal through a resistor 35.
The precision voltage of 4.35 v. is applied to the + input terrninal of amplifier 36. , Amplifier 36 receives power for operating from power supply 12. A resistor 37 , provides a feedback path from the output terminal of amplif,ier 37 to its input ' terminal. The precision voltage input to the + terminal creates the necessary DC ' component in the output signal of amplifier 36. Resistor 37 is selected to provide substantial linearity in the amplification of amplifier 36 over the range of the input signal, here from about 2.0 v. to 6.5 v.
The input signal in converter circuit 11 is processed in the same way ', and by similar circuit components. The input signal is applied to a first of the two terminals of capacitor 32. Resistor 40 connects the second of the capacitor 32 terminals to ground. The cathode of diode 45 is connected to the second terminal of ,~,~
capacitor 32 and the cathode of diode 45 is connected to a voltage summing point 47. ~
The inverted signal is processed in the same manner, with a capacitor ,, 33 connected by a first of its two terminals to the output terminal of amplifier 36 and ,~
by the second of its two terminals to ground through resistor 41. A diode 44 has its ', cathode connected to the second terminal of capacitor 33 and its anode connected to the surnming point 47. Both capacitors 32 and 33, resistors 40 and 41 and diodes 44 and 45 should have values as equal as possible. A capacitor 50 has a first terminal connected to summing point 47 by resistor 48, and a second terminal connected toground; Diodes 44 and 45 along with resistor 48 form the full wave equivalent of the ,;' diode circuit of Figs. 1 and 2. I prefer the value of resistor 48 to have an order of magnitude approximately two times smaller than that of resistor 40 or 41 to assure that the major portion of the charge formed on capacitors 32 and 33 is transferred to capacitor 50. At the same time, the value of resistors 40 and 41 should be smallenough to discharge capa,citors 32 and 33 relatively quickly if amplitude of waveform a shrinks~' The output signal is provided as a negative current through resistor 56 to an output tenninal 58. A low impedance load 51 connected between terminal 58 andground receives the output signal and also provides a discharge path for capacitor 50 in a manner congruent with that explained for the circuit of Fig. 1.
Operation of the converter circuit 11 of Fig. 4 is essentially the same as for the circuits of Figs. 1 and 2. Capacitors 32 and 33 quickly charge to the DC

WO93/12387 ,~ PCr/US9Z~

component value of 4.35 v. When the low frequency periodic input signal indicative of flame is present, capacitors 32 and 33 have their second terTninals driven to below ground on the less positive half cycles of the input and inverted signals respectively. .
The voltage at summing point 47 is thus continuously driven negative when the 5-15 -S hz. frequency characteristic of fla ne is present in the signal at point 20, and capacitor 50 charges so that its first terminal is negative. This charge is constantly bled by the discharge path of resistor 56 and load 51, and replenished by current supplied by capacitors 32 and 33. When the periodic component of the signal on path 29 va~ishes, the current to charge capacitor 50 is no longer replenished and the voltage at the first terminal of capacitor 50 rises to near zero. The load 51 senses this change in ~-the mag~itude of current flow through terminal 58 and will typically respond by `~
shutting a fuel valve or taking some other safety-related action. It is intended that the load 51 comprise the circuit described in my co-pending application entitled "Fail-Safe Condition Sensing Circuit", although there are obviously other possible candidates for load 51 as well.
There was earlier mention made of the output of the level shift detector 19 on path 21. The voltage on path 21 rises abruptly should flame be detected to ,~
- have suddenly been extinguished. The voltage on path 21 is conducted to terminal 58 ' j ~rough a diode circuit comprising a series connected resistor 53 and a diode 54.Such a sudden rise in voltage on path 21 will cause the voltage on terminal 58 to rise toward 0 v. as well, simulating loss of the periodic voltage amplitude on path 29. In this way, either of the two conditions indicating loss of flame will cause a rise in the voltage on terminal 58 which may be used by the load to close a valve or take other acthities required by the no flame condition. I
The preceding discussion has to some extent emphasized the function of sirnulating a flame rod output with an infrared detector. There is the equally important factor of using for safety critical functions, circuitry which produces an output signal of one polarity in response to an input signal of the opposite polarity and while powered from a source of the same opposite polarity. The reader should therefore expect that the designs described abovè show only what is for my -~
application the more convenient orientation of the polarities. It is likely that some applications may in the future use the opposite orientation for the polarities. This means that the diode orientation throughout the circuits will be reversed in order to be compatible with this reversed polarity. There will certainly be other variations on this ' invention as well, all of which will fall within its spirit and which l wish to include within the scope of the claims which follow.

Claims (5)

The preceding has described my invention. What I wish to claim is:

1. A converter circuit (11) for providing an output signal of a first polarity responsive to a periodic low frequency input signal varying between twovoltage limit values of a second polarity different from the first polarity, comprising:
a) a first capacitor (32) having a first of first and second terminals connected to receive the input signal;
b) a first resistor (40) connecting the second terminal of the first capacitor (32) to ground;
c) a first diode (45) having first and second terminals with the second terminal thereof connected to the second terminal of the first capacitor (32), said first diode (45) orientation being a function of the input signal's polarity;
d) a second resistor (48) having the first of first and second terminals connected to the first diode's (45) first terminal;
e) a second capacitor (50) connected between ground and a second terminal of the second resistor (48) and providing the output signal at the second resistor's (48) second terminal; and f) a resistive path (51, 56) connected to discharge the second capacitor (50), wherein the invention is characterized by g) a level shift detector (19) sensing the input signal voltage and providing a relatively large output signal voltage having the second polarity responsive to a rapid change in the input signal voltage, and a relatively smalloutput signal voltage otherwise; and h) a diode circuit comprising a second diode (54) and a third resistor (53) in series connection, said diode circuit connecting the level shift detector (19) output to the second resistor's (48) second terminal with said second diode in said diode circuit oriented to conduct the relatively large output signal voltage of the second polarity from the level shift detector (19) to the second resistor's (48)second terminal.

2. The circuit of claim 1, further comprising a filter circuit (23, 24, 25) transmitting the input signal to the first capacitor's (32) first terminal.

3. The circuit of claim 2, wherein the filter circuit (23, 24, 25) includes a low pass filter (24).

4. The circuit of claim 3, wherein the second diode's (54) anode is connected to receive the output signal of the level shift detector (19).

5. The circuit of claim 1, wherein the second diode's (54) anode is connected to receive the output signal of the level shift detector (19).