Simple Flip-Flop Switch With Momentary Switch

Or , Fun withThyristors- this circuit came about through trying to make a footswitch controller board for some music software , a looper , which will be the subject of another Instructable. I'll show you the circuits that led up to the Flip-Flop so you can see how it works more easily. It contains only a few parts , and is dependable- there is no need for debouncing circuitry.

First , a basic thyristor switch. A thyristor like the one here , a T709 , works like a transistor and a latching relay .It has three pins , Anode , Cathode and Gate. You can put a large voltage across the Anode and Cathode and it will not pass any current between them until a small voltage is applied to the Gate , at which point current will flow. Unlike a normal transistor however , the current continues to flow once the Gate voltage is taken away , and won't switch off until the current-carrying circuit is broken in some way.

So , the momentary switch is closed , a voltage is applied to the gate , and a current is allowed to flow through the thyristor and the LED which lights up.The momentary switch is released but the current keeps flowing until the circuit is broken by disconnecting the battery. (Or you could have another , Normally Closed , momentary switch in the circuit to break it when pressed).

The interesting thing here is that when the momentary switch is pressed , the gate shorts the power through the Cathode to earth , and although the Anode/Cathode is now set to allow current to flow between them there isn't enough current flowing through to light the LED , it's only on release of the switch that power is returned and sufficient current flows through the thyristor and hence the LED.

The second circuit adds two of that first basic circuit together. When one switch is pressed it lights its LED on release. Now when the second switch is pressed the shorting of the power to earth also shorts out the thyristor and LED in the first circuit and that first thyristor switches off and the LED goes out , when the switch is released the second thyristor conducts and the second LED turns on. And so on.

This is the Flip-Flop circuit. Again , there are two circuits comprised of a thyristor and an LED. Instead of each of the resistors in the LED circuits however , there is a relay coil belonging to a double coil bistable latching relay.

These relays have two coils , and each coil controls a switching function . In different relays this switch may be configured in different ways , in this case when a current passes through coil one , the one marked with pink , it closes the switch between the pink dots , When a current passes through the second coil marked with blue , it closes a switch between the blue dots. One or other of the switches is always closed until the coil corresponding to the open switch is activated and the situation is then reversed.

There is only one momentary switch controlling both LED circuits this time.

Now if we start by assuming the internal relay switch is closed on the pink side ,when the momentary switch is pressed Thyristor 2 has a voltage applied to its gate and the blue coil is put in circuit . BUT the relay coil is not activated until sufficient current is actually flowing through it , which as we have seen previously only happens on the switch being released.

When it is released , the current flows , the LED 2 turns on and the blue coil is activated and it closes the blue switch.

When the switch is next pressed , Thyristor 1 has a voltage applied to its gate , it is switched on but not with enough current to light the LED or activate the relay coil until the switch is released , when it puts the switch back to being closed on the pink side and lights LED 1.

And so on.

The fact that the thyristor only allows current to flow after the momentary switch to the gate is released is the reason the circuits are activated alternately- otherwise you'd probably just hear the buzz of the relay switching from one coil to the other at speed , if it worked at all.

This Flip-Flop wasn't designed from the bottom up , just discovered because the bag of thyristors I bought worked in this way combined with the latching relays I had. Other thyristors with other specifications may not work. The critical factors are the required gate voltage and the amount of current needed to keep the thyristor conducting . The relay coils also have requirements for the voltage / current required to activated .

The thyristors I used were T709 , phase control thyristors. I can't find them for sale any more , and can't find an equivalence database to give alternatives. I've ordered some other phase control thyristors to see if they can be used. I'll edit this Instructable if I learn more.

Update :

I tried using a thysistor that was cheap and easily available on ebay , the MCR-100 , £2.50 for 50 from Hong Kong , and it works fine. The circuit I found to be temperamental when part of a larger circuit , so I'm trying this one which takes the diodes out of the thyristor / relay coil circuit and has them (or any other load) being controlled by the second switch in the relay. I'll see how this one goes , it's fine on the breadboard (except the relay pins don't like breadboard . At all.)

Because the LED's are no longer in series with the relay coils , they will need a suitable resistor in series otherwise they will burn out.