Electronic – Proper operation of latching relay

All this hanging contacts around the relay coil can and does work .The red and green start and stop buttons seen in machine shops are older than me .There is another way that uses the fact that the pull in voltage is always much greater than the dropout voltage .Get your relay and a DC power source thats the same as the coil volts .The relay coil will off course pull in reliably.Now place a series resister thats the same as the coil resistance in your relay coil power supply circuit.The coil now only gets 6V and wont pull in .If you momentarily short the resister the coil will pull in .If you momentarily remove the power the relay will drop out .So you can make a normal relay have a bistable latch function .The time to do this rather than the orthodox approach is when you need the contacts for other things and you are not worried about power.I have tried this with small DC relays but have not done this on AC relays.

As I think you understand, for this model of the relay there is only one coil and set and reset are carried out by polarity reversal. This is indicated on the schematic symbol.

Figure 1. Coil polarity for set and reset operation.

The problem is with the ULN2003.

Figure 2. The ULN2003 inverter and a detail of one output stage.

Notice that this is an open-collector transistor output stage. It can only pull low (sink) and cannot source (supply) current.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 3. (a) What you created (in top schematic). (b) What you need - H-bridge.

You need a little redesign. A simple H-bridge would do the trick.

I don't know which coil voltage you are using but it may be possible to drive the relay coils directly from the micro-controller. Calculate the current required from the coil specifications on page 8 of the datasheet. Then compare this to the maximum source and sink currents of the micro-controller. Watch the chip total current too. You might be constrained to energising one coil at a time.

^{simulate this circuit}

Figure 3. Snubbing options. Option (b) integrates the four diodes into a bridge rectifier saving some PCB space and soldering.