JPH02130431A - Pyroelectric type infrared sensor - Google Patents

Abstract

PURPOSE:To prevent the generation of an induction noise by connecting a resistance in series to both of a drain electrode and a source electrode of an EFT within a shield case or to the source electrode alone. CONSTITUTION:In a pyroelectric type infrared sensor which houses a pyroelectric element 1 and an FET 2 in a shield case 6, resistance rD and rS are connected in series to, for example, both of a drain electrode D and a source electrode S of the FET 2 respectively. Thus, the resistances rD and rS work on a high frequency induction noise as impedance larger than a reactance component. Thus, these resistance can act on a low frequency infrared detection signal as DC-oriented resistance element.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a pyroelectric infrared sensor using a field effect transistor (FET). As this type of infrared sensor, the one shown in FIG. 2 is conventionally known. In the figure, 1 is a pyroelectric element, 2 is an F
ET and Rg are gate resistances, 3 is a drain lead wire, 4 is a source lead wire, 5 is a ground lead wire, and 6 is a shield case.

7 is an infrared transmitting window. 8 is a power source connected to the drain lead wire 3. FET2 has a gate electrode G, a drain electrode D, and a source electrode S.

According to this configuration, depending on the change in infrared energy received by either of the pyroelectric elements 1 through the infrared transmission window 7,
A change in the amount of charge occurs. The source current is controlled by the change in the gate potential caused by this, and this change can be detected as a change in the source voltage between the Rs terminals by providing an appropriate source resistance Rs outside the sensor.

In such a pyroelectric infrared sensor, the lead wires 3, 4.5 outside the shield case have an inductance component with respect to high frequencies, and generate induction noise due to external high frequency radio waves. In FET2, a feedback capacitance Cf is formed between the gate and drain electrodes in the high frequency region, and due to this feedback, high frequency noise generated at terminal 3, for example, affects the gate potential and causes a drop in the source voltage detection circuit outside the sensor. There is a problem of giving a false signal.

The present invention has been made in order to solve the problem of generation of false signals from sensors due to high frequencies as described above. FIG. 1 shows an embodiment of the present invention, and the same parts as in FIG. 2 are given the same reference numerals. In the figure, resistors rD and r are connected to the drain electrode and source electrode S in the shield case.
5 are connected in series.

According to this configuration, the resistor r'D+r8 acts as a large impedance due to the actance component against high frequency induced noise, but acts as a DC resistance element against a low frequency infrared detection signal of about 1 m. Therefore, the resistors ro and r are effective against high frequency noise induced in the lead wire 3 and input to the drain electrode and high frequency noise amplified by the FET output from the source electrode S, which are the main causes of erroneous signals from the sensor. 's has a suppressing effect, and the erroneous signal appearing on the lead wire 4 as a sensor output is significantly reduced. The resistance values rD and rs are selected so as to suppress the voltage effect of the detection output to a small value within a practical range.

Table 1 shows the results of a sensor error signal generation distance test using transceivers of 430 MHz, 3 W output and 825 MHz, 3 W output, which demonstrate the effects of the present invention.

In Table 1, in this test, resistor paste was used as the resistors rD and r3, and Ω was set as 1 to 4, but other resistor types and resistance values may be selected depending on the structure and application of the sensor.

Also, although not shown in the table, there is a resistance r, only at the source electrode.
Even when connected with g, the influence of external high frequency radio waves can be reduced.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention, and FIG. 2 is an explanatory diagram showing a conventional pyroelectric infrared sensor. 1...Pyroelectric element 2...FET Drain lead wire Source lead wire Ground lead wire Shield case Infrared transmission window

Claims (1)

[Claims] A pyroelectric infrared sensor uses a pyroelectric element and a field effect transistor and houses them in a shield case.
A pyroelectric infrared sensor characterized in that a resistor is connected in series to both the drain electrode and the source electrode or only to the source electrode of the field effect transistor.