A Novel Electronic Latch Design

This is a very simple circuit (fig 1). Four optocouplers are used - the lowest one performs the latching function - when its LED is 'on' it will turn on its transistor which keeps the LED burning. Turning on the LED is achieved by moving the switch to the 'on' position momentarily. The latch is switched off by shorting the LED current through the switch by moving it to the 'off' position. The switch needs to be biased to the 'centre-off' position (ie, a spring returns the switch to the position where both pairs of contacts are open). Alternatively, two push buttons could be used to pull the respective nodes down - if both buttons are pressed simultaneously then the circuit will be forced into the 'off' state.

The transfer ratio of the optocoupler needs to be at least 100% otherwise it just won't latch. I've found the Isocom ISD203 (current transfer ratio > 200% for LED current 10 mA) works well, although any other with similar performance should be just as good. A useful feature of this latch is that it always powers up in the 'off' state - unlike those cross-coupled transistor bistable things which just do what they damn well want.

circuit diagram of the latch
Figure 1: Basic Latch Circuit

The component values shown here allow transistor and LED currents of around 20 mA. An inverter function can be achieved by diverting the current from a 'normally on' LED through an optocoupler transistor so that, in this case, when the main LED chain is conducting the rightmost LED is turned off. The diode currents need to be controlled fairly accurately to ensure that the transistors saturate - and the transistor currents need to be limited to ensure saturation. This means that the on LED currents have to always be the same as the on transistor currents.

Adding LEDs in series provides outputs (I've included an indicator (red) LED to show when the circuit is 'latched' - this can be omitted if not required as can optocouplers that aren't required - but remember that the series resistor values need to be recalculated). All outputs can be 'two-state', 'open-collector' or just 'floating' if required. Open collectors allow a 'ORing' functions to be produced - by connecting two or more collectors (and emitters) together (figure 2a). An 'AND' function can be produced using the circuit shown below in figure 2b:

circuit diagram of the OR gate
Figure 2a: OR Circuit


circuit diagram of the AND gate
Figure 2b: AND Circuit

The switches in the basic latch circuit can be replaced by optocouplers (figure 3) - the 'ON' coupler operates like the switch - the latch changes state (assuming it is initially off) as soon as the LED conducts. The 'OFF' optocoupler changes the state of the latch (conducting to non-conducting) by cycling the optocoupler LED current from zero to 20 mA and back to zero - the state changes when the current returns to zero. This is unlike the case where the switch is used - the latch changes state as soon as the switch is closed. The difference arises because the opto transistor diverts current from the lowest LED only, whereas the switch diverts the current that would otherwise flow through all the diodes in the chain. The opto transistor cannot be connected across all four LEDs because the transistor current would then be 54 mA or so. This would require a similar input LED current to ensure saturation.

Latch Using Optocoupler Inputs
Figure 3: Latch Using Optocoupler Inputs



ISD203 datasheet.

Last updated: 9 January 2003;   © Lawrence Mayes, 2002/03