# Circuit problem - two sensors need to be activated to power on

Discussion in 'Digital Circuit Design' started by seanspotatobusiness, Sep 17, 2016.

Sep 17, 2016
31
0
I made a circuit for a lamp that takes input from two PIR sensors (HC-SR501) and turns on some LEDs. Two transistors are used to amplify the signal so the LEDs are switched on fully. They're actually five sets of five LEDs in series in parallel (25 total) which isn't shown in the diagram below - the voltage supplied is 15 V. There is also a capacitor involved to make the circuit fade on and off. The problem is that the circuit only comes on fully if both PIR sensors are connected and activated. If either is disconnected or connected and not activated, the circuit lights up only very dimly. In the diagram below, the green wires are signals from the PIR sensors. I guess a single PIR sensor cannot send enough current and this causes the signal voltage to sag and prevents proper function. Is that correct? Edit: Actually, that doesn't make sense; I don't think much current can be drawn through that resistor anyway.

2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,575
389
Hi Spud,
If that is a 10K Base resistor from the PIR output, it is a to high value, try a 1K
E

EDIT:What is the Vout high voltage from the PIR.?

Update:

Are those Green wires the outputs from the PIR's, if so they appear to be linked together, which will cause a problem, in fact it is acting like and an AND gate. So both PIR's will have to be High for the it to work.

Last edited: Sep 17, 2016

Sep 17, 2016
31
0
Hi!

I'll try lowering the value of that resistor. The Vout from the PIR is meant to be 3.3 V. I'll measure this to check.

I figured that the green wires being connected in this way may be acting like an AND gate but the thing is, I still have a problem if I disconnect one of the green wires. I think I can fix the AND gate problem but adding a diode to the middle of each green wire. I have some with 0.4 V Vf. But this doesn't solve the issue of why it lights with both PIRs connected and activated but not with one (like I say, I tried with one green wire disconnected).

4. ### AlbertHall Well-Known Member

Jun 4, 2014
2,250
445
Reduce the resistor value but use two of them, one for each PIR.

5. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,575
389
hi S.
If the PIR is like the type I use, I get about 3v when activated, the output resistance of the PIR is about 2k, it will only supply approx 1.5mA.
What are the transistor types.?
E

6. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,953
1,131
If you are trying to push 30mA through each LED and the PIR can source only 1.5mA then you might need to replace each transistor with a Darlington type to get enough current gain.