This is set-reset latch: it is a 'personality module for 4QD-TEC transistorised relay system. However it is an R-S latch with a difference in that it can be set by a High or a Low signal (as appropriate) on either input. The latch can be forced eithr way, and stays that way until next changed.

The circuit

Very simple and at first it looks like a plain old 2 transistor bistable. But it's a bistable with dome subtle differences. The transistors have built in current limits so that their collectors can be forced to supply rail or to zero volts, without damaging the circuit. Moreover, if you thus short an input - the circuit will latch into that state and stay there.

So the circuit has two inputs, pins 3 and 4.

• To set the latch:
  • High on pin 3
  • or Low on pin 4
• To reset the latch:
  • High on pin 4
  • or Low on pin 3

This facility greatly enhances the circuits versatility in, for instance, wiring a machine where combinations of highs and/or lows can be used. Or course, you must make sure the switches cannot simultaneously put a high and a low on the same input: this would short out the power supply!

A full description of the circuit, with values, is given on part two in the 4QD-TEC members private area.