4QD-TEC Electronics Circuits Reference Archive
Forcible latch module - Part 2

In case you have not read it, Part 1 of Forcible latch module

The Circuit


How it works

¨But that's just a plain old 2 transistor bistable - what's clever about that?¨ I hear you ask. There is a little more to it than that. The idea is to turn the base relay into a latching relay, but one with two inputs. Either input may be used to set/reset the latch so one is 'high to set'/'low to reset' the other is 'high to reset'/'low to set'

The transistors of the latch are fitted with a current limit of around 10mA. If you want to read more about current sources see Current Sources and Mirrors. By fitting the 47R resistors, the transistors are turned into 47R current sources. However as with most current sources, they operate as normal transistors until that current is reached.

The 4K7 collector resistors can only supply about 2.5mA (with a 12v supply). So, if for instance pin 3 is momentarily shorted to ground, Tr2.s collector is low, Tr1 is turned off. Tr1's collector rises and keeps Tr2 turned on.

If you then momentarily short pin 3 to supply, Tr2 can only take 10mA so its collector goes high turning Tr1 on. When the external signal is removed, Tr2 will stay off as Tr1's collector is low.

So either input can be forced high or low. It's very simple idea and a very simple circuit but one which, in the right circumstances, could be very useful.

The only other point to notice is the two capacitors connected to the collector: these give some measure of noise rejection but more importantly they define the latch's state at power on since C2 will take longer to charge than C1 so the circuit will always turn on with Tr2 conducting.

Other relevant pages

Transistorised relay driver has a connector to accept this latch.
Current Sources and Mirrors

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