On-- off-- on-- off timer circuit

Question

I want to make a circuit to do something that is relatively simple but I need to find the optimal way to do it in order to take as little current as possible. Here is what I want to do:

When a trigger is activated, it launches a cycle. A cycle is:

1. 1 second "on" (let's say it closes a relay)

2. 1.5 second "off" (opens the relay)

3. 1 second "on" (closes the relay)

4. Turn "off" and then wait for the event to initiated the circuit to happen again. (relay is open).

The time "on" for step 1 and 3 needs to be the same but they are not necessarily the same as in step 2. The trigger that activates the circuit will probably be a short pulse. I thought about using one or two 555 timers and counters but I am not sure exactly how and not sure if that is the best way.. What would be the best way to do something like that to use as little current as possible? The circuit will run with batteries(4*AA or one 9V)...

Answer 1

Based on your need for arbitrary on and off times and a small fixed cycle, as well as low current, then as mentioned in the comments, a micro controller will do what you need with minimal code.

Pseudo code

While(1){
    sleep;
}

Interrupt on input high to low{
    output on;
    wait 1000ms;
    output off;
    wait 1500ms;
    output on;
    wait 1000ms;
    output off;
}

Answer 2

This is something you want to do with a microcontroller. Get one that uses very little power when sleeping, like a 16F PIC with the XLP (extra low power) feature.

After initialization, the micro goes to sleep. It is then woken by a particular edge on a pin. Make sure no current is required to keep the pin in the unasserted state. For example, if using a pullup, have the high to low transition wake the micro. That means during the long off time when power consumption matters, no current goes thru the pullup.

When the micro is woken, it produces the on-off-on-off sequence on one of its output pins, which is used to drive a relay. During that time, power consumption in the micro doesn't matter much since it will be swamped by any relay you can find. Even so, running a XLP PIC from its internal oscillator will take very little current.

When the micro finishes the on-off-on-off sequence, it goes back to sleep looking for the next wakeup event.

If the sleep time is long and even tiny amounts of current matter, use a BJT to turn on the relay, rather than a logic level FET. Both will work fine, but the BJT will likely have less leakage. Some FETs, particularly those that turn on with low gate voltage, can have surprisingly high leakage currents. This is just a general guideline, and you have to check the datasheets of any particular parts you consider using.

Answer 3

As pointed out, your best option is an MCU. You didn't point out your level of experience in the area so I don't know if you have access to an MCU programmer or not.

If not, I would suggest looking into commercially available MCUs with pre-configured/installed boot loaders/programmers (i.e. Arduinos) that could easily achieve the task you are looking to complete.