Transistor triggering as soon as powered

Projects General Guidance
1

I am making a security system with sensors that communicate with a central control box using HT12E and HT12D RF. I have the communication working, in the sense that when I switch on the sensors it immediately sends the message to the control box to trigger the alarm. however, when I tried to limit the sending of the signals to when a sensor is triggered, it still sends the signal as soon as its powered. The sensor is connected to the base of a 2n2222 transistor, which is supposed to trigger the signal only when the sensor is triggered. I have tested the Signal pin on the PIR motion sensor used with a multimeter and it is outputting about 0.64 volt even when the sensor isn't triggered. here is the schematic:

sensor_schematic
sensor_schematic722×659 27.4 KB

Any help would be appreciated.

1 Like
2

You don't seem to have any base resistor in that circuit so the first thing I would try is to put a 10K resistor between the sensor and the base.

Also make sure that pin AD11is configured as an input and has the pull up resistor enabled. If you can't then add a 10K resistor from the transistor's collector to +3V3.

Thanks for providing a real schematic with your problem Karma point for that.

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If it can be configured :slight_smile: ...

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the ht12e isnt a microcontroller. its in ic for encodign radio signlas

5

See the update to my initial post.

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i added the base resistor and it seems to be working. im going to wait for teh sensor to calibrate then will find out

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i added the base resistor but it doesn't seem to be activating

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You need the pull up resistor Mike mentioned.

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ok i'll add 10k from 3.3v to collector alogn side base resistor

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Are you sure the PIR output is active HIGH, and the transmitter is active LOW? Because the transistor will invert the logic.

11

yes im pretty sure according to the datasheets

12

Can you post a link to the data sheets please.

Use Rosc = 1.1MΩ as recommended in the datasheet, you have it as 51K which looks way out.

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If the PIR sensor is a HC-SR501, then it's OK to not use a base resistor.
Because those sensors already have a 1k5 resistor in series with the output.
Leo..

14

You are spamming the 433MHz frequency, *TE low means you are transmitting continuously.
https://www.holtek.com/documents/10179/116711/HT12A_Ev130.pdf

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