Beam Breaker reverse opertion

[Ehobby]

I am building an electronic pendulum. I want to use a 2N2222 to power the drive coil when the pendulum bob interrupts an IR beam.

The 100 ohm, the 1K ohm, and the red led will be removed and an air core coil will be substituted.
The coil consists of a plastic sewing machine bobbin fully wound with 30 gauge magnetic wire. Resistance 10 ohms, inductance unknown. The coil is bridged using an led to suppress the back EMF caused by the collapsing field.

The circuit as shown works BACKWARDS. The 2N2222 turns on when the photodiode is illuminated and turns off when the photodiode is not illuminaed.

For simplicity the source of the ir beam is not shown.

How can I change the circuit to make the 2N2222 turn on when the ir beam is interrupted and off when the off when the beam is not interrupted.

photodiode circuit

PHOTODIODE BASIC CIRCUIT.jpg (21.17 KiB) Viewed 10433 times

[I_Daniel]

Well here is a rather untested and probably incorrect answer. Have you tried replacing the 2N2222 NPN transistor with a PNP transistor?
Just thinking but what turns an NPN on should turn a PNP off.

[pebe]

Hi Ehobby,
Are you building this for the fun of it or because you need one. If you need one you can get one on ebay, complete with an analogue clock innards that runs on 1.5V for around £5UK.

If you are building for fun, then using a continuously running IR emitter and sensor is not the most economical way of doing it. And I am not at all sure that the circuit you posted will work - I think that the output current of a photo diode would be in the microamp range and not enough to drive the 2N2222 to power an electromagnet.

[Ehobby]

Thank you. You are right, the photodiode output is too low and the problem is a little more complicated.

I am building this circuit for my own amusement
After a lot more searching I came across a circuit that comes close to doing what I want.
I breadboarded it and it works as is, but I still have to make some changes to make it power a drive coil.

Here's the URL describing the circuit.
http://www.buildcircuit.com/dark-sensor ... hotodiode/

Once again, thanks

[pebe]

But you still need a source of light to keep it working. What happens at night?

You might be better off with a small magnet attached to the pendulum that operates a reed switch as it swings near it.