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12 Volt PIR to Timer

Hello ETO,

Have been working on circuit that uses a PIR (motion detector) to initiate a 555 timer to turn
on 12 volt LEDs for a couple of minutes. Have queried this excellent forum previously
and gotten some good feedback. Previous post on this system:
The latest change had been to
upgrade Q2, the transistor handling the load, to a heavier AUIRF540Z.

Thought the installation of the PCB and the four 12 volt LED lights was completed and a method
for permanently connecting the AC adapter to the house mains was the last step.

The installation worked for fifteen days and stopped. Also have two test units working on the bench
and they crapped out too.
Using a digital multimeter the obvious culprits were checked, the power to the PIR, the ground and the
V+12 volts at pins one and eight of the 555.

The indicator LED on the PCB is not coming on so that leads to the idea that the timer
is not working. Because they are not soldered into the board unit tests of the PIR
and 555 chip are normal.

This seems to indicate the failure is at Q1, the transistor between the PIR and the 555.

Have studied the datasheet on the PIR, HC-SR501. Its says the output is 3.3V but it
does not mention the current. Could the output from the PIR be overloading the BC547?

It is realized that troubleshooting a circuit via email is not ideal. But any ideas
on how to find the defect would be greatly appreciated.


Allen in Dallas


Well-Known Member
Most Helpful Member
If Q1 has failed then it suggests excessive base current. Add a 1k resistor in the base connection of Q1 to limit the current.



Well-Known Member
Most Helpful Member
IF it's a genuine HC-SR505. the module should already have a 1K resistor in series with the output as supplied, and the makers give a direct base connection as an example.

Clone or counterfeit devices (or modified genuine ones) may be different, of course. An extra base resistor would not do any harm either way.

Edit - Check which way around you are fitting T1? If that is backward it could explain a lot.
I'd suggest always using the proper outline on overlays to avoid such possible errors.
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