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.