Hi,

I've built a few small relay type circuits before but this one seems a little more complex.

What I am trying to archive is a circuit where a switch when turned on would turn a motor forward for approx 3 seconds and then turned off, turn that same motor backwards for approx 3 seconds.

Ideally the circuit should use no power when the switch is off - after the motor has stop.

Thanks in advance.

 

What type of motor? DC or AC?
Is the timing automatic or do you mean the operator switches on for about 3 seconds and then off?

The circuit needs a "bi-stable" action and if you want it to remember its state with no power then a special kind of relay will be needed.
This is called a latching relay. It has a magnet so that once energised, the contacts remain in this position until the supply is reversed.

Something like this.

 

Thanks JDT,

I haven't brought the motor but it will be DC and have a 6v battery (4 x AA's) and consume approx 150mA.

The logic should be switch on then motor forward 3 seconds then stops for any durtion (likely 1-2hrs), switch off then motor reverses for 3 seconds then stops with ideally no power consumption at this point

I was thinking something like (switch closed-power on) runs a motor as it chargers a capacitor. once charged power to the motor is cut. Then if power from the battery is cut(switch open) capacitor discharges and in turn runs the motor. I've built something like this before but I'm thrown on how to have the motor run in reverse on the discharge.

The circuit should not remember state but should turn the motor in the opposite direction when the battery is removed so I don't think it would use latching relays.

Thanks again

 

Would it help if you thought in terms of a momentary double throw switch instead of "on" and "off". A simple timer could be created from a capacitor and transistor to hold a relay for 3 seconds. So flip the switch one way and a timer holds the relay closed and powers the motor one-way, flip the switch back and a timer/relay powers it the other way.

 

Problems can arise if you try to reverse a motor instantaneously. You should allow time for the motor to come to a stop before you reverse it.

 

The logic should be switch on then motor forward 3 seconds then stops for any durtion (likely 1-2hrs), switch off then motor reverses for 3 seconds then stops with ideally no power consumption at this point.

I think the 1-2 hrs would qualify as a full stop.

 

That circuit, while commendably simple, would require an enormous cap of at least 200,000uF (0.2F) to give a 3 sec run time at a constant 150mA. The run time would also be very load dependent.
Here's a circuit which should do what you want:


Edit: The relay coil draws current the whole time the switch is closed. A slightly more complicated circuit could ensure the coil current is only for the 3 sec motor run period.

 

Thanks Alec_t,

Especially for the level of detail you've provided. I'm going try building this to get a better understanding of it. One question what component is the U1a?

Also I would love to hear how it could be more efficient by only charging the relay for the run time. Ideally I want to make the circuit consume a little power as possible to make the battery last at least 6 months - assuming 1 cycle a day.

 

what component is the U1a?

CD4070B (as shown). This is a 4-gate package. You need only 1 gate for the circuit I posted.

Edit: Here's a revised circuit to reduce the current consumption. It now uses 2 of the gates in the IC. I've also added some suppression components to connect directly across the motor.