Electric components to toggle DC motor polarity

[alec_t]

The timer is a digital timer with a 2-wire 12v output

Is the timer output a push/pull type or open drain/collector type, or what? Can you post a link to its datasheet?

 

[cowboybob]

If my understanding is correct - all those latch relays still need *reverse* polarity to un-latch them

No, they don't. Power is applied to initiate the relay and the latching is accomplished by the way it is wired. It is then "un"-latched by removing power.

Two examples (Click for full size):

Or one even simpler (perhaps simplest...):

 

[dcwatson84]

Argh, another hiccup ...

KISS - I do like that design - but I forgot another limitation of the timer... It's designed to run motors for short durations (not hold relays open) so the duration is limited to something short like 30 seconds. Right now It comes on twice, once in the morning to open the door, and once in the evening to close it, and I was assuming that pattern going forward. Your design is more robust, so I may have to scrap my timer to implement it, but Im also not sure how it would impact the battery life with it constantly needing to hold current to a relay. I'll have to investigate that.

Edit - If I use latching relays I think the timer only needs to hit the relay once to start the cycle. Then again to cut the power off?

 

[dcwatson84]

No, they don't. Power is applied to initiate the relay and the latching is accomplished by the way it is wired. It is then "un"-latched by removing power.

I thought that was the operation of a standard relay? I thought "latching" relays required simply a pulse of power, at which point the latch remain connected: https://en.wikipedia.org/wiki/Relay#Latching_relay
And then it is unlatched by a pulse of reverse polarity.

 

[cowboybob]

There are latching relays that are mechanically latched when powered (even briefly) and, as such, need a reversal of current to overcome the latch and return to an unlatched condition. Rarely used anymore. The last one I saw was in an OTIS elevator control box built in the 1920's.

The first schematic I posted does, indeed, continue to draw some power (albeit, not much - just the low power Day/Night relay) when in its powered state.

That's why I really like the second design: it shuts itself off. And no electrical reversals at all.

 

[MaxHeadRoom78]

I thought "latching" relays required simply a pulse of power, at which point the latch remain connected: https://en.wikipedia.org/wiki/Relay#Latching_relay
And then it is unlatched by a pulse of reverse polarity.

These type latch and stay latched magnetically when power removed and unlatch with a reverse polarity pulse, IOW they have memory, you can also get the same thing in solenoids if needed.
Max.

 

[spec]

Hy DC,

Here is my offering for controlling your chicken coop door. I have done a number of designs, but I think that this is the simplest of the bunch with good performance and flexibility. If you favor this approach I will list the exact parts. If you insist on using a single timer a small circuit can be added a to take care of that.

Issue 02 of 2016_07_23

ERRATA
(1) 'TIMER 1 DOWN' should read 'TIMER 2 DOWN'
(2) 'COUP' in all instances should read 'COOP'

NOTES
(1) This is an outline schematic and has not been optimized or evaluated in depth.
(2) The design uses two independent timers, one for door opening and one for door closing. A single timer with two independent channels would also be suitable.
(3) The motor is controlled on a time basis and there are no limit switches or stall current sensing.
(4) Limit switches can be added if necessary.
(5) It may be necessary to limit the motor stall current. A simple test will decide.
(6) The two relays are standard 12V single pole change over types that are freely available and reasonably priced.
(7) Timers are also freely available and reasonably priced.
(8) If the timers switch on period overlaps, the motor will stop and the circuit will not be damaged.

 

[MaxHeadRoom78]

Hy DC,

Here is my offering for controlling your chicken coup door. I have done a number of designs, but I think that this is the simplest of the bunch with the best performance and flexibility.

​

​

I admit I have skimmed through the previous posts so I may have missed something, you don't think it prudent to place a limit switch for extreme travel? You are just relying on time?
Max.

 

[spec]

I admit I have skimmed through the previous posts so I may have missed something, you don't think it prudent to place a limit switch for extreme travel? You are just relying on time?
Max.

Yes, I think so but obviously not sure at this stage. Some motors are fairly happy being stalled, but it may be necessary to limit the stall current, which should not be too difficult. Limit switches may be necessary in the end.

Of course, the best way to do this job is with an Arduino, which with a real time clock (RTC) shield and motor H bridge shield could do the time keeping and control the motor. The whole shooting match would probably cost under £10UK and could even be programed to make the chickens a cup of tea in the morning.

spec

 

[MaxHeadRoom78]

If limits were needed as a safety feature they can both be inserted into the two motor conductors with a reverse biased diode across each.
Max.

 

[dcwatson84]

I admit I have skimmed through the previous posts so I may have missed something, you don't think it prudent to place a limit switch for extreme travel? You are just relying on time?
Max.

Yes, I think so but obviously not sure at this stage. Some motors are fairly happy being stalled, but it may be necessary to limit the stall current, which should not be too difficult. Limit switches may be necessary in the end.

Of course, the best way to do this job is with an Arduino, which with a real time clock (RTC) shield and motor H bridge shield could do the time keeping and control the motor. The whole shooting match would probably cost under £10UK and could even be programed to make the chickens a cup of tea in the morning.

spec

As for the price - I've looked up the parts and I have yet to see a single suggestion that could be done for anywhere under $40? Especially given that I would need to place contacts down by the door, which would have to be weather proofed. (but maybe I just dont know the best places to look for the parts?)

I think in an ideal world I'd have all the bells and whistles, but this is a chicken coop door . Im not worried about extreme travel, because there's really no way its going to happen, and the consequences are not severe. If the timer is set for ~10 seconds, then the door would have to travel for 20+ for it to run out of space, which is just very unlikely. The only result of minor over-travel would be that I'd need to re-calibrate it, which is fine every few months because the timers have to be re-set anyway to account for daylight hours.

 

[spec]

If limits were needed as a safety feature they can both be inserted into the two motor conductors with a reverse biased diode across each.
Max.

Yeah, that would do it.
I did consider contactless limit switches which would be better for the environment: hall effect, magneto resistance, etc. It all depends on what the OP says though.

spec

 

[MaxHeadRoom78]

For those to replace the simple N.C. limits I would think will require extra wiring, hall effect usually open collector or? etc.
e.g. the hermetically sealed Honeywell microswitch type can usually be picked up on ebay etc.
Max.

 

[spec]

For those to replace the simple N.C. limits I would think will require extra wiring, hall effect usually open collector or? etc.
e.g. the hermetically sealed Honeywell microswitch type can usually be picked up on ebay etc.
Max.

Ah yes- good idea.

spec

 

[Les Jones]

If you are prepared to use limit switches here is a suggested circuit.

This is how it works. When power is applied C1 generates a positive pulse at the junction of D1 and D2. The pulse passes to both relays via D1 and D2 but only the relay that is not at its limit position pulls in. The relay that is pulsed on then holds itself in via D3 or D4. When the limit is reached the relay then drops out. The value of C1 and R2 will have to be adjusted to give a suitable length of pulse to suit the relays. The circuit will behave in an unpredictable manner if power is applied when neither limit switch is operated. Another method that could be used if you are prepared to use a microcontroller is to put a sensor on the drive to the rope drum that generates one or more pulses per revolution. These pulsed increment a counter when the motor is driving up and decrement the counter when the motor is driving down. The system has to initialy be tought the count that coresponds with the up and down positions. I use this method for controlling a roller blind and it has been working for years without a problem. (Let me know if you would like the schematic and code. It uses a PIC).

Les.

 

[KeepItSimpleStupid]

The more you know, the less you don't know.

See: **broken link removed**

A dual coil magnetic latching relay basically has a set and reset coil. When the polarities are correct, a momentary pulse is all that's required to change state, Single coil latching relays require a polarity reversal. The dual coil is basically two single coil latching relays.

The circuit above could probably be adapted to operate on a +12 V pulse rather than a pulse to ground.

==

The automotive relays are cheap.

**broken link removed**

https://www.parts-express.com/te-co...n-relay-spdt-20-30a-(formerly-bosch)--330-070

The microswitch I linked to was <$2.00 at Allied Electronics and here's some other ones: **broken link removed**

 

[MaxHeadRoom78]

One advantage with the limits in the two motor conductors it does not require any extra, or at most very minimum wiring.
Max.

 

[KeepItSimpleStupid]

One advantage with the limits in the two motor conductors it does not require any extra, or at most very minimum wiring.
Max.

A disadvantage is that the limit switch usually sees the full motor current.

 

[MaxHeadRoom78]

The OP states that the current is .8 A at 12-14 V.
I suspect that the locked rotor current would not exceed the L.S. rating, never the less I have used this method for larger motor than this on press back-gauges etc.
Max.

 

[KeepItSimpleStupid]

I've used this method too and I'm speaking in the general case. The limit switch may have to be carefully selected to prevent failure.