Is it possible to control a latching (bistable) relay with a single MCU pin?

Question

Controlling a latching relay, like HFD2/012-S-L2 (datasheet [PDF]) with two MCU pins is straightforward:

And here is the corresponding code:

void switchRelayTx() {
    HAL_GPIO_WritePin(GPIOA, GPIO_PIN_0, GPIO_PIN_SET);
    HAL_Delay(10);
    HAL_GPIO_WritePin(GPIOA, GPIO_PIN_0, GPIO_PIN_RESET);
}

void switchRelayRx() {
    HAL_GPIO_WritePin(GPIOC, GPIO_PIN_15, GPIO_PIN_SET);
    HAL_Delay(10);
    HAL_GPIO_WritePin(GPIOC, GPIO_PIN_15, GPIO_PIN_RESET);
}

But when you need >= 10 relays in the project you end up out of MCU pins very soon. Thus I wonder if it's possible to control such a relay with only one MCU pin, maybe by utilizing a High-Z pin state, and without too many additional external components? Off the top of my head, I couldn't come up with the schematic.

Simply putting an "I2C pin number extender" like PCF8574 is possible of course, but I'm curious if there is a little less brute-force solution. Using a regular relay is not an option in the project because of the extra current such relays consume. Using a single-coil latching relay could be an option though.

Answer 1

Using a 3-state pin is a workable approach. Use a pair of comparators to detect when the pin is below 1/3 VCC for one coil and above 2/3 VCC for the other coil. Then your code defines 3 states: drive low, drive high or no drive. The I/O would be tied to a midpoint voltage.

The LM339 has an open-collector output with enough current for the coil, so it would replace the transistor.

Answer 2

Using discretes you can use N+1 pins since you can multiplex if you want the set coils engaged or the reset coils engaged.

Common negative all set coils into an P-channel and all reset coils into an N-channel. If you drive the set/reset high, n-channel will conduct. if you pull it low the p-channel will conduct.

The you enable the common high side switch for the relay you want to change. This could be done with a CD4010 for example.

Or if you can afford to change the relays to 5V and can afford a +5V and -5V power supply sufficient to switch the relay you can do the above in a tri-state buffer chip or shift register. As hacktastical and lundin already hinted.

Using other chips you can use h-bridges or some MAX4820.

Answer 3

It possible, but inconvenient. You could consider using chips such as 74HC138 to control one relay at a time. For example 2 chips would allow you to control 8 relays with 5 control pins- 3 address plus S/R strobes. Or 7 pins to control 16 relays (that could be reduced).

You would use PNP rather than NPN transistors since that chip has inverted outputs. Ask if this is not clear how to implement.