GB2305318A - Schmitt trigger for DTMF receiver - Google Patents

Abstract

A Schmitt trigger 10 comprises an NPN transistor T1 and a PNP transistor T2, with T2 supplying positive feedback to the base of T1 through resistor R1. The Schmitt trigger, which has a large hysteresis threshold and which switches rapidly, may be used in a DTMF receiver for remote control of a VCR or a cable box, or in an infrared receiver detector.

Description

SCHMITT TRIGGER The invention relates to a Schmitt trigger, in particular to a Schmitt trigger built from two transistors.

Conventionally, a Schmitt trigger is built from an operational amplifier the output of which is positively fed back to the positive input of the operational amplifier. This feed back generates an improved noise immunity and shows a corresponding hysteresis threshold (see Horowitz + Hill: "The Art of Electronics", Cambridge University Press, 1980, p. 126ff).

Such a Schmitt trigger configuration uses an integrated circuit and is therefore easy to realize, but high in cost.

Further, Fig. 3 shows a known discrete-transistor Schmitt trigger using two transistors Q 1 and Q2 (Horowitz + Hill: "The Art of Electronics", Cambridge University Press, 1980, p. 127). Those two transistors Q 1 and Q2 share a common emitter resistor r2 and are connected with respective collector resistors r3 or rl, respectively, to the power supply line. It is essential that the Q1 collector resistor is larger than Q2. In this way the threshold to turn on Q1, which is one diode drop above the emitter voltage, rises when Q 1 is turned off, since the emitter current is higher with Q2 conducting. This produces hysteresis in the trigger threshold, just as the above mentioned integrated-circuit Schmitt trigger.The known discrete Schmitt trigger shows the problem, that in its "Off' state the second transistor Q2 is always conducting which leads to a small but not negligible power consumption. Further, the velocity of the output transition is not sufficient.

It is another disadvantage of the known Schmitt trigger circuits that the hysteresis threshold is small. In fact, it is possible to realise a large hysteresis threshold, which is necessary for certain applications by reducing the ratio of rl relative to r3 and r2. But in this case the collector output voltage of transistor Q2 will be increased when it is switched on.

This may not be appropriate for logic "low". More stages for level translation become necessary in that case.

It is therefore an object of the invention to provide a low-cost discretetransistor Schmitt trigger with a fast transition from one state to another with a large hysteresis threshold to compensate for DC offset with AC coupling to non-linear load to improve the noise immunity, said Schmitt trigger having no standby current at logic "high".

This object is solved by the subject matter of independent claim 1.

Further advantages and preferred embodiments are described in the dependent claims.

The Schmitt trigger of the present invention comprises two transistors T1, T2, wherein the second transistor T2 is positive fed back to the base of the first transistor T1. The feedback of the second transistor of the Schmitt trigger T2 is accomplished with a resistor R1. The input signal of the Schmitt trigger is level shifted with an average DC at zero potential with a voltage divider R2, R3, wherein normally the input signal of the Schmitt trigger is AC coupled.

To realise a hysteresis threshold larger than in known systems, the ratio of of resistor R1 relative to resistors R3 and R2 must be reduced. The collector output voltage of of transistor T2 will be increased when it is switched on.

A Schmitt trigger according to the invention can be used in a DTMF to PWM conversion apparatus, which does a simple conversion of dual-tonemultiple-frequency signals to pulse-width modulated signals.

The Schmitt trigger according to the invention consisting of two transistors configured with a positive feed back realizes a fast transition from one state to the other state with a large hysteresis threshold to compensate for DC offset due to AC coupled signal to a non-linear load and improvement of noise immunity.

The operation of a Schmitt trigger according to the invention is as follows: When the positive peak of the input signal, which is AC coupled and level shifted with an average DC at zero potential, exceeds the predetermined threshold, Von, the transistor T1 becomes active. The base voltage of T2 is pulled low, which causes T2 to be active, which further causes T1 to be saturated very quickly. This positive feed back will cause fast saturation of both transistors, so that a fast switching time is realized in a cost effective way.

In a similar way, the positive feedback ensures that both transistors switch off rapidly, when the signal input falls below a predetermined threshold, Voff. When T2 is turned on, the input voltage Voff must be lower than Von, wherein the difference is the hysteresis which provides improved immunity against the noise interference.

The "On" and "Off' voltages can be calculated in the following simple way: Von = Vbe (R2 + R3) / R3

R2 / /R3 R2 + R3 // R1 V,ff = Vbe - Vcc * R1 + R2//R3J R3//R1 wherein Vbe : Base-emitter voltage (normally about 0,55V) Vcc :Supply voltage R2//R3: means (R2*R3)/(R2+R3) R3//R1: means (R3*Rl)/(R3+Rl) This results in: R1R2 + R2R3 + R1R3 R2 V,ff = Vbe * - Vcc * R1R3 R1 This simple and cheap Schmitt trigger can be used in various applications, for example in a DTMF to PWM conversion apparatus, and in an infrared receiver detector, etc. where an improved noise immunity and an adjustment of the detection threshold to compensate for DC offset is needed.

A preferred embodiment of the discrete-transistor Schmitt trigger and its use in an DTMF to PWM conversion apparatus will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows a circuit diagram of the Schmitt trigger according to a first embodiment of the invention, Fig. 2 shows the use of a Schmitt trigger according to fig. 1 in an DTMF to PWM conversion apparatus, and Fig. 3 shows a known discrete-transistor Schmitt trigger with two transistors.

In the following description of a preferred embodiment values of used components are given in brackets. It may be mentioned that the invention itself is not restricted to these components.

Fig. 1 shows a discrete-transistor Schmitt trigger, with a transistor T1 (BC 848), the base of which is coupled to a voltage divider consisting of two resistors R2 (4.7kOhm) and R3 (4.7kOhm), which normally receive the input signal IN through a capacitor C (4.7uF). The voltage divider R2, R3 is grounded like the emitter of the transistor T1. The collector of the transistor T1 is positively fed back with a second transistor T2 (BC 857), i.e. the collector ofT1 is connected to the base of T2 via a resistor R4 (43kOhm). Via resistor R5 (43kOhm) the connection point between the base of T2 and the collector of T1 is connected to a supply line Vcc (5V), as is the case with the emitter of T2. Transistor T1 is of NPN type, whereas transistor T2 is of PNP type. The collector of T2 is positively fed back via a resistor R1 to the base of the first transistor T1. The output OUT of the Schmitt trigger is formed by the collector of the first transistor. The functional description of this Schmitt trigger has been already given in the first part of the description, so that it is omitted here. Further the equations for computing the different voltages have already been given.

Fig. 2 shows an embodiment using the Schmitt trigger of fig. 1 in a DTMF to PWM conversion apparatus, wherein DTMF signals are converted into PWM signals. Such an apparatus mainly consists of a switch circuit 11, followed by an amplifier circuit 12 and Schmitt trigger 10, the output of which is fed into a microcontroller 13, which evaluates the TTL signals of the Schmitt trigger 10 as a function of the DTMF frequencies.

The purpose of this apparatus is to program the different channels of, for example, a VCR or a cable box via a telephone line. Therefore in this application an electret-type microphone (not shown) is provided at the input IN of the apparatus, wherein the switch circuit 11, which is controlled by the microcontroller 13, provides power to the microphone for operation. Such an Electret-type microphone is preferred over dynamictype microphone, as the gain is higher and it is more cost-competetive.

The dual tone frequency signals of the telephone are picked up by the microphone and amplified by the amplifier circuit 12. The Schmitt trigger 10 converts those signals into TTL signals suitable for the microcontroller 13. The output OUT of the microcontroller 13 programs e.g. the cable box following (not shown). With this apparatus the various channels of a TV set, a VCR or a cable box can be programmed from, for example, a remote cable station via a telephone line.

Fig 3 shows a known Schmitt trigger comprising two transistors Q 1 and Q2, wherein the base of Q2 is fed back to the collector of Q1 via a resistor of 10kOhm. The collector of Q 1 is connected via resistance r3 (1.5 kOhm) to a supply voltage Vcc whereas the collector of Q2 is connected via a resistance rl (1.0 kOhm) to Vcc. The emitters of both transistors are coupled via a resistance r2 (100 Ohm) to ground. The operation of the known Schmitt trigger has been already described in the introductionary part of this description.

Claims (8)

1. Schmitt trigger comprising a first transistors (T1) and a second transistor (T2), characterized in that the second transistor (T2) is positive fed back to the base of the first transistor (T1).

2. Schmitt trigger according to claim 1, characterized in that the type of the second transitor (T2) is complementary to the one of the first transistor (T1).

3. Schmitt trigger according to claim 1 or 2, characterized in that the second transistor (T2) is fed back via a resistor (R1).

4. Schmitt trigger according to one of the preceding claims, characterized in that the input signal is level shifted with an average DC at zero potential with a voltage divider (R2, R3).

5. Schmitt trigger according to claim 4, characterized in that the voltages are calculated as follows: Von = Vbe (R2 + R3) / R3 R1R2 + R2R3 + R1R3 R2 Voff = Vbe * - Vcc * R1R3 R1

6. Schmitt trigger according to anyone of the preceding claims, characterized in that the input signal is AC coupled.

7. DTMF to PWM conversion apparatus comprising a Schmitt trigger (3) according to claim 1 to 6.

8. DTMF to PWM conversion apparatus according to claim 7, characterized in that the apparatus further comprises a switch circuit (11), followed by an amplifier circuit (12) and a microcontroller (13), which also controls said switch circuit (11) wherein the Schmitt trigger (10) is located between the amplifier circuit (12) and the microcontroller (13).