Recently, I wanted to try an electronic scarecrow in my lab window to scare off some birds (often nocturnal) that make a lot of noise and dirt. Although there are many options for building one, I quickly setup a battery-powered eternal flasher using just four parts taken from my junk box. It is still unclear if a burst of light is effective, but you can see my project details here.
Well, let us start with its incredible circuit diagram!
Yup, just four jellybean parts:
- BC547B NPN Transistor x1
- 100uF/40V Electrolytic Capacitor x1
- 22KΩ ¼ W Resistor x1
- 3mm (or 5mm) Red LED x1
Seems a bit odd, this pretty small circuit but requires 18VDC for operation. Not to worry, just wire two 6F22 9V batteries in series to make the power source (9V x 2 = 18V) and you are ready to go…
The idea was first tested on a tiny breadboard, and the final built up was on a flake of perfboard. Since the current draw is negligible, the battery pack should last for several weeks, long enough for the birds to seek and find a new roost.
Regular visitors of this site will be familiar with the NPN relaxation oscillator circuit concept given here (https://www.codrey.com/electronic-circuits/avalanche-pulse-generator-an-introduction/).
In this circuit, a sawtooth signal with circa 1.2Hz can be found across the 100uF capacitor, which is however extremely dependent on the working voltage. The discharge current surge is strong enough to operate a regular LED with it. The idea works for me with the BC547B transistor (https://www.farnell.com/datasheets/410427.pdf).
What is acquired about the function is that the NPN transistor contains a type of zener diode between the base and the emitter. As observed, my transistor behaves like a 10.4V zener diode and the setup needs at least 14VDC to initiate the flash. You can certainly experiment with another NPN transistor and the operating point (working voltage) until it oscillates.
OK, now you know how I pieced together a little electronic scarecrow for my personal use from parts I had around. This is an extremely low current flasher circuit that lights up a standard light emitting diode for a split second eternally. The whole thing runs off a pair of budget batteries, and the run time is expected to last several weeks. Since this project does not demand any exotic or expensive parts, give it a try. Have Fun!