Optical Rat Trap Trigger

A simple circuit to make a better rat trap

Rat Trap Schematic

This circuit can be used to trigger a trap when a rat or other animal interrupts a beam of infrared light. You can use this to build your own trap, or to modify an existing one. I used it to modify a wire cage type of live rat trap.

The advantages over a simple mechanical trigger are:

The disadvantages are: I used a four pack of NiCd C cells which delivers about 4.8V and has about 1500mAH capacity. These keep the trap functional for more than a week on a single charge. NiMH AA cells would be smaller and have about 1/3 greater capacity, but I used the C cells because I already had them and their size is no problem for my trap. If you decide to use a power supply instead of batteries, I strongly recommend a fuse located away from the trap because rats like to chew the wires and cause shorts.

Circuit Notes

Place S1 such that the the main circuit is energized when the trap is set, and the buzzer is energized when the trap is sprung. this will cut power to the circuit so that it will not continue to drive the motor or consume power after the trap has sprung. BUZ1 should be attached to a few feet of wire so that it can be placed where you can hear it when the trap is tucked away inside a cabinet, crawl space, attic, etc.

Place LED1 and Q3 facing each other on opposite sides of the trap at whatever location you want the sensing beam. Mine are about 8" apart. If you need them spaced much further than that, you'll probably have to make some circuit changes to accommodate it. Since one of my goals was to keep power consumption low, LED1 is supplied with as little current as I can give it without causing unreliable operation. I have no specific recommendation for the type of IR LED or phototransistor to use here. I just grabbed some unmarked and unknown ones out of my junk box and they worked. You may have to change resistor values or make other adjustments for your specific IR LED and phototransistor.

LED2 just indicates when the trap is armed. It turns off when the trap is sprung. It can also. You might have to reduce the value of the series resistor in that case. In my experience so far, the rats don't seem to be bothered by the presence of the glowing red LED, in case you're wondering.

When the light from LED1 illuminates Q3, Q3 turns on and provides current to the base of Q4, which sinks the current that passes through R1, LED2, and LED1, to ground, which keeps Q1 turned off. When the beam is interrupted, Q3 turns off, which turns Q4 off, which allows the current that flows through the LEDs to flow through the base of Q1, turning it on, and in turn turning on Q2 which provides power to the motor.

D1 protects Q2 from inductive "kick" from the motor. Optimally, this should be a fast recovery diode like the FR107 that's shown. If you don't have any fast recovery diodes, then common 1N400x series diodes are better than nothing.

Q2 has to be able to handle enough power to drive the chosen motor. 2N4401, 2N2222, PN2222, etc. are adequate for many small motors, especially since it will normally be turned on only for a fraction of a second. Larger transistors can be used if you need to drive a larger load. For Q1 and Q4, more common 2N3904's or PN2222's will probably work in place of the 2N4401's, but I haven't tried it so I can't promise.

C1 is for EMI suppression and should be soldered directly accross the terminals of the motor. Some motors may already have a capacitor there, or a pair of caps, one between each terminal and the motor housing. In those cases there's no need to add another one.

I used a small gear motor from my junk box. What you should use depends on what kind of trap you're building or modifying, and possibly what you have lying around. Using a solenoid instead of a motor is also a possibility. The only real requirements are that it be able to operate adequately from the battery voltage, and that it not draw more current than Q2 can provide.

This thing sort of evolved by trial and error. If I were to start from scratch, I think I'd rearrange it so that the final transistor is an NPN because NPN power transistors are more readily available. Not a big issue though, since the 2N4403 is adequate for my particular motor. And since the trap works fine, I'm not inclined to change it. I've caught several rats already.

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