Experiment 8: A Relay Oscillator
60 Chapter 2
Experiment 8: A Relay Oscillator
You will need:- AC adapter, breadboard, wire, wire cutters and strippers.
- DPDT relay. Quantity: 1.
- LEDs. Quantity: 2.
- Pushbutton, SPST. Quantity: 1.
- Alligator clips. Quantity: 8.
- Resistor, approximately 680Ω. Quantity: 1.
- Capacitor, electrolytic, 1,000 μF. Quantity: 1.
Look at the revised drawing in Figure 2-63 and the revised schematic in Figure
2-64 and compare them with the previous ones. Originally, there was a direct
connection from the pushbutton to the coil. In the new version, the power
gets to the coil by going, first, through the contacts of the relay.
680
12v
DCFigure 2-64. The oscillator circuit shown in schematic form.Now, when you press the button, the contacts in their relaxed state feed power
to the coil as well as to the lefthand LED. But as soon as the coil is energized, it
opens the contacts. This interrupts the power to the coil—so the relay relaxes,
and the contacts close again. They feed another pulse of power to the coil,
which opens the contacts again, and the cycle repeats endlessly.
Because we’re using a very small relay, it switches on and off extremely fast.
In fact, it oscillates perhaps 50 times per second (too fast for the LEDs to show
what’s really happening). Make sure your circuit looks like the one in the dia-
gram, and then press the pushbutton very briefly. You should hear the relay
make a buzzing sound. If you have impaired hearing, touch the relay lightly
with your finger, and you should feel the relay vibrating.
When you force a relay to oscillate like this, it’s liable to burn itself out or de-
stroy its contacts. That’s why I asked you to press the pushbutton briefly. To
make the circuit more practical, we need something to slow the relay down
and prevent it from self-destructing. That necessary item is a capacitor.12v DC from
AC adapterFigure 2-63. A small revision to the previ-
ous circuit causes the relay to start oscil-
lating when power is applied.