# Small hobby electronics build question.

Hello,

I’m wanting to build a simple device out of parts I can by at the local hobby electronics store. I want to send a 4V pulse down a 3rd wire when another 4V circuit turns off. I hope it can be done using small hobby diodes and capacitors available for pennies.

My application is to sense when a LED circuit turns off, on a flying drone taking photos. The drone’s front LED goes out when the shutter of it’s camera is activated. If i can generate a small pulse in a secondary circuit when the LED circuit turns off then I can capture the timing of the camera event.

Is this a simple construction? How do I achieve this?

You can use a fairly simple transistor circuit to invert the signal from the LED. When the LED voltage goes low, your transistor output would go high, and vice-versa.

You take your LED driver voltage, run that through a base resistor to limit the current going into the base of the transistor, ground the emitter, and have a resistor connecting the transistor’s collector to your +4 supply. The output comes off of the transistor’s collector.

When the LED is on, current flows through the transistor (base to emitter) turning it on. This makes the transistor conduct electricity, effectively shorting out the voltage divider created by the transistor and the resistor from collector to your +4 supply. When the LED turns off, the current stops flowing into the base of the transistor, the transistor turns off, and has a much higher impedance than the resistor on the collector, so the collector resistor pulls the output high.

The resistor values are not critical. You want the base resistor low enough that current from the LED driver will drive the transistor completely into saturation, but not so low that the base current will damage the transistor. On the output side, you want the collector resistor to be large enough that when the transistor switches on, the output voltage will drop close to zero, but low enough that when the transistor switches off that the output voltage floats up close to the supply voltage.

The output voltage will never go all the way to zero or to +4.

Make sense?

If you want something more complicated, a 555 timer circuit or maybe a small and cheap microcontroller will be a better solution. Microcontrollers are dirt cheap for some of the smaller ones, require little external circuitry, and can be programmed to do all sorts of complicated responses to inputs. The down side is that you need to be able to program them.

A CMOS 555 timer in monostable operation might work. See Figure 3 on Page 8. The Trigger Input would monitor the voltage across the LED.

Hello and thanks for the replies.

I should have added that my little device is powered by the LED circuit so when it goes out, so does power to my device. It’s last dying task is to send that pulse with whatever residual power is left in it’s circuits. Also when the LED powers on again in a second or so it should do nothing.

Sorry I didn’t include this initially. Perhaps it makes this device’s task impossible.

I’m totally green when it comes to electronics, and I initially thought of a small capacitor and some diodes. Power up the capacitor through one diode and another diode directing it away to my timing pin when the voltage collapses, but I realise this is just a short to my pin. Perhaps it’s not a simple problem that can be solved with discrete components.

A discrete capacitor can definitely store enough energy to power your timer/transistor/microcontroller for a few seconds.

If you connect a capacitor to one leg of the LED, and a wire to the other, you will get a pulse when the LED turns off. And another (but in the opposite direction) when the LED turns back on later.

The pulse won’t be 4V: it will only be the LED voltage. You can make it 4V if you can find the driver pin for the LED: the LED is probably driven by a 4V signal through a resistor…

To suppress the reverse-polarity pulse when the LED turns on again, you can short it out with a diode.

The length of the pulse you get from your coupling capacitor will depend on how big the capacitor is, and how much current is taken by whatever you connect to the other end.