I’ve been working for some time now on a clock pulse generator that uses relays rather than transistors or integrated circuits. This is to go along with the relay computer that Zevin and I showed this summer. Currently -shhhh- there’s an LM555 timer IC circuit hiding up top. So how do you make a clock circuit out of relays? My first impulse is to design the circuit around the relaxation oscillator model.
My tinkering began with a trick that cheap high voltage people use to make a crude oscillator. Connect a relay’s normally connected switch contacts in series with the power to the coil.
The cycle begins with the relay pole in contact with the upper normally connected contact. An instant later though the pole is pulled away from this contact by the draw of the electromagnet which has since energized. Another instant later, since the flow of current is broken, the electromagnet loses its grip on the pole, and the pole returns to the normally connected contact. The very same process begins again, and continues over and over.
This circuit I disdainfully call a “buzzer.” It buzzes along at a rapid and irregular rate that can’t be controlled. But it is a beginning, as it is a crude form of a relaxation oscillator.
If we take a same circuit but add only one more component we will have a much more useful timer.
Behold a timer with a much slower and stable oscillation. This circuit worked well in my tinkering. It produced clocks ticks anywhere from a 10th of a second to 20 or 30 seconds. This is a good solution for a timer that operates in these frequency ranges. However the circuit has serious limitations. For some combinations of coil inductance and capacitor the back EMF resulted in irregular periods.
Also for larger capacitor values the period did not increase accordingly. The capacitor doesn’t discharge all the way when the relay pole disengages from the normally connected contact. Note in the simulation below that the voltage of the capacitor never drops below about half of the maximum. To get the full delay from the capacitor it needs to be fully discharged before the beggining of every cycle.
A second relay is needed functioning as a one shot timer. The second timer allows the capacitor to fully discharge in the downswing of the cycle.