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Time Delay Relay Help!

Discussion in 'General Electronics Discussion' started by stoggy, Oct 20, 2013.

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  1. stoggy


    Oct 20, 2013
    Hi guys, new to this forum but not new to electronics. While I'm not a wizard I do know my way around a bit. I require some help with time delay relays and making them work in a circuit I have designed.

    The Project: Power folding side mirrors

    The mirrors use two wires (power and ground) to unfold the mirrors via a motor. To fold it simply swaps power and ground. I've designed a relay logic circuit using two standard 5 pin Bosch automotive relays that will swap power and ground no problem.

    My problem is I need a third relay to activate one or the other relay. See attached diagram:


    So just a refresher in case there is any questions, the way these relays work is as follows:

    Pins 85 and 86 are the switch (polarity does not matter)
    Pin 30 is power in and is normally closed to accessory pin 87a
    Pin 87 is the other accessory

    I've used the relays backwards by using Pins 87A and Pins 87 as my inputs and Pin 30 as my accessory output, this way I can switch between power and ground as Pin 87A on both relays is always grounded while only one relay can switch at any time. Pin 85 is my switch.

    Here is where I need this forums help. I want the mirrors to unfold when they get the accessory (hereafter called ACC) signal from the ignition switch and fold in when ACC is dropped. I did a bit of reading on time delay relays but not sure which one is suitable.

    How I see it: Once the time delay relay receives ACC trigger it sends power to Pin 85 on Unfold Relay for 5-10 seconds then kills the output. Once the relay loses the ACC trigger it sends power to Pin 85 on the Fold Relay for 5-10 seconds then kills the output.

    Perhaps there is a simpler way of doing this altogether, perhaps it's a bit harder than I think.

    Any help is appreciated!
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi stoggy and welcome to Electronics Point :)

    Thanks for the thorough and clear description.

    Do the mirrors need the voltage to be applied for a limited amount of time? Or do they have limit switches so that when the mirror reaches the end of its travel, the motor stops automatically?

    If the mirrors have limit switches, the control system can be very simple. You just swap over the connections to 87 and 87A on one of the relays, and drive both coils (85) from the ACC signal.

    With 87 and 87A reversed on one relay, the relay logic will connect the 12V battery voltage to the mirror in one direction when the relay coils are de-energised, and in the opposite direction when the coils are energised. There is no timeout, so the system relies on the limit switches in the mirrors to cut power to the motors when the mirrors are fully extended or retracted.

    If the mirrors need timed pulses, I can design a circuit to do it, but you will have to build it up on stripboard and it will contain quite a few components. Based on the resourcefulness you've displayed already with your post, I don't think you'd have any difficulty with that. In that case, can you provide a part number and/or a link to a PDF data sheet for the relays.
    Last edited: Oct 21, 2013
  3. stoggy


    Oct 20, 2013
    The mirrors do have stops, however I don't believe they are limit switches. I put a clamp on the positive wire and it draws 1 milliamp when the motor stops. I would rather have a pulse signal for open and close.

    If you have time to design a circuit I'd really appreciate it.
  4. stoggy


    Oct 20, 2013
    The mirrors do have stops, however I don't believe they are limit switches. I put a clamp on the positive wire and it draws 1 milliamp when the motor stops. I would rather have a pulse signal for open and close.

    If you have time to design a circuit I'd really appreciate it.

    Here's another thought....since the mirrors do actually stop (ie: the motors do not continue to try and turn the mirror) could I not use timed relays instead of the ones I'm using? I'm not convinced I need a fancy circuit or anything.

    Wha I have in the diagram does work as long as a third relay is used where 12v goes to pin 30 and 87a is connected to the fold relay. When ACC is sent to 85 it sends power to the open relay. This works perfect except when you start the car the computer drops ACC until the engine is running and then kicks ACC back in.

    What this causes is the mirrors to unfold and then fold back in temporarily then unfold all the way in a period of about 2-3 seconds. Could a capacitor hold enough power to tie the circuit over?

    I might just be running in circles here lol
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    When the mirror is fully extended or retracted, if the current drops to 1 mA and the motor stops, there are limit switches, not just stops, and I recommend the arrangement I just described. It's equivalent to what you described using a changeover relay to switch +12V between 85 on the unfold relay and 85 on the fold relay, but it avoids the extra relay.

    The problem you describe during cranking may be best handled differently. The problem is that during cranking, the 12V supply will not be stable. Depending on where it's taken from, it will certainly be low during cranking, it may have high voltage spikes on it, and it may be very uneven.

    This might work: connect a relay with a lower coil voltage (e.g. 9V or even 6V) from the starter control to chassis, and have it interrupt the +12V line to the mirror driving relays. So while the car is cranking, the mirrors can't do anything.

    This relay may be suitable:
    It's rated for 9V but pulls in at 6.1V. It will survive 12~14V on the coil for short durations typical of cranking.

    If you really want only a pulse, you can use two delay relays. You need a third relay that's driven from the ACC signal, which switches +12V between the two delay relays, and each delay relay drives the coil of one of your two relays in your diagram.

    So you need the following relays:

    K1 standard SPDT automotive relay
    K2 delay relay activates for 5~10 seconds - for unfold
    K3 delay relay activates for 5~10 seconds - for fold
    K4 standard SPDT automotive relay - the "UNFOLD RELAY" in your diagram
    K5 standard SPDT automotive relay - the "FOLD RELAY" in your diagram.

    ACC to K1.85
    K1.86 to chassis
    K1.30 (common) to +12V
    So K1 directs +12V to 87 while ACC is ON, and to 87A while ACC is OFF.

    K1.87 to K2.input
    K2.output to K4.85

    K1.87A to K3.input
    K3.output to K5.85

    So each time K1 changes state, either K2 or K3 becomes energised and generates a pulse of 5~10 seconds on either K4.85 or K5.85.

    The rest of the connections to K4 and K5 are just as in your diagram.

    I don't know how well this arrangement would work during cranking. Some circuitry could be used to detect cranking (through the 12V supply rail dropping below say 9V) but as you say this is not simple. So I think you would be best with a simple non-pulsed changeover circuit and a relay to disconnect the supply during cranking.
  6. stoggy


    Oct 20, 2013
    Ok I understand a little better now. I will use the first way you described with swapping the connections on one relay and just applying ACC to both coils.

    So after some digging and probing the switches in my car it seems that once the key switch goes past ACC it reaches Ignition (IGN). ACC and IGN can be on at the same time with the engine off. Go further on the key switch and it cranks over the engine. While cranking it drops ACC and IGN until the engine is running and the key returns to IGN.

    Is there a way to just hold over the voltage from the key in ACC position through to the engine cranking and back to IGN position. Can a capacitor do this?
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    When you say that it "drops" ACC and IGN, do they go to zero volts?

    In older, simpler cars, ACC and IGN remain energised, but the voltage on them dips because of the heavy load on the battery due to the starter motor.

    But I guess you have a car that deliberately drops ACC and IGN to zero during cranking so that (a) there is no other load on the battery during cranking, and (b) devices powered from those supplies don't see a dirty supply voltage. Do you know this for sure?

    A capacitor will hold a charge but probably not for long enough, depending on exactly what you want to do.

    If you want your wing mirrors to continue to unfold while you're cranking the car, you will need the capacitor to provide the energy to the mirrors themselves. This will require a pretty big capacitor, depending on how much current the wing mirrors draw when folding/unfolding. Personally I would accept that the mirrors stop moving during cranking then resume when cranking stops, but if you want to look into using a capacitor, here are the calculations.

    A charged capacitor provides a voltage that drops steadily as you draw current out of it. The formula for this behaviour is:

    dV/dT = I / C
    dV is the change (delta) in voltage, measured in volts;
    dT is the time period, in seconds;
    I is the current being drawn from the capacitor, and
    C is the capacitance, in farads! (Not microfarads.)

    This rearranges to C = I dT / dV.
    So let's say your capacitor is charged to 13V and needs to remain over 10V to operate the motors in the mirrors properly. dV = 3V.

    Say the mirrors take five seconds to unfold, and you might crank the engine for more than five seconds, so we need the capacitor to supply the mirrors for five seconds. dT = 5 seconds.

    Say each mirror draws 0.5 amps (can you find out how much they actually draw? It could easily be more than that), and the two relay coils (which also need to be powered) draw 0.1 amps each, for a total of 1.2 amps. I = 1.2A.

    C = I dT / dV = 1.2 x 5 / 3
    = 2 farads!

    This is just a ballpark figure because I don't know the actual current drain of the mirrors.

    You can buy a 1.5 farad 16V electrolytic here: and it's only USD 247!

    Supercapacitors are also available in high capacitances but not with suitable voltage ratings.

    It MIGHT be possible to use several supercaps in series or parallel, but you are still looking at big bucks. Another option would be rechargeable batteries; you could probably use ten or twelve C or D cells. Personally I would just accept that the mirrors stop moving during cranking!
  8. stoggy


    Oct 20, 2013
    The car does drop ACC and IGN to zero volts.

    The reason I wish to have the mirrors open on ACC is if I'm sitting in a parking lot listening to music the engine would probably be off and if only be running in accessory mode (ACC) but would still like to be able to see out my side mirrors. The other reason for wanting to hold the voltage over from ACC to actually starting the engine is that these mirrors fold/unfold in under 3 seconds, they're rather quick.

    From ACC to engine running on the key switch is less than 3 seconds so I just don't want the mirrors flapping about while starting the engine I hope this makes sense?

    I will measure the current draw on the mirrors tomorrow. I'm going to guess it's under an amp, it's wires are 18-20 awg, can't handle much amperage at that size.

    I hate to pick at the relay thing again, sorry it's what I'm good at. I'm a heavy duty mechanic and specialize in custom truck wiring and pretty everything I do involves relays everyday. Now, I'm not afraid of having several relays.... The setup I have designed uses three and it works but it doesn't hold the output closed if the trigger drops out.

    What type of delay relay would I require and can I use one with multiple contacts to cut down on the amount of relays required? There's a lot of variations from what I've been reading and it's pretty overwhelming.

    If you were to design a pulse circuit for me what would you charge? I had an oxygen sensor simulator built a while back that basically oscillated the voltage back and forth from 0.1v to 1.3v every 3 seconds to satisfy the computer requirements. It involved 555 timers and capacitors and diodes. Would my circuit contain similar commonly available items?

    Thanks so much for your help thus far. I'm learning a lot!
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I don't understand your second paragraph. With the design I'm suggesting, the mirrors WILL unfold on ACC. When ACC goes ON, they unfold, and when ACC goes OFF, they fold up.

    The only problem is during cranking, when ACC disappears; my suggestion was to disable the +12V during cranking, using a third relay with a normally closed contact that is activated (to open its normally closed contacts) during cranking. During cranking, there would be no supply voltage available to the mirrors, so they wouldn't move. This solution does require access to the starter contact on the ignition switch though.

    This would prevent the mirrors from flapping about when you're starting the engine. It would also mean that if they were in the process of unfolding when you started to crank the engine, they would stop moving, and they would finish unfolding when the engine was running.

    The type of delay relay I was thinking of is the kind that closes for a settable time when voltage is applied to it, then opens. But I don't think a delay relay will actually help, because the delay will be started by a change in ACC (from OFF to ON, or from ON to OFF), and these will occur at the start and end of cranking, so the delays will be retriggered anyway.

    Your mention of multi-pole relays reminded me that I was going to suggest that to replace the two relays in your first design. Since I suggested that you swap the 87 and 87A connections on one of them, and drive the coils in parallel, obviously you could replace them with a single relay if it had two changeover contacts on it.

    This contact arrangement is called DPDT (double pole, double throw) or DPCO (double pole changeover) in the electronics world. There are six contact connections. You can also get DPDT switches, which can be wired as a "reversing switch", which is exactly what you would be doing. It's the same as the two separate relays with 87 and 87A swapped on one of them, but using a single unit.

    So if suitable automotive DPDT/DPCO relays are available, you can replace the two that drive the mirrors with a single unit.

    As long as you have access to the starter contact on the ignition switch, I think it's simplest to use it to activate a relay to interrupt the +12V to the mirror, so they just stop moving during cranking. During cranking the 12V rail will drop significantly; this is why I recommended a relay specified for 9V that will activate with only 6V on its coil. That does mean that the relay's specifications would be exceeded if the 12V rail is actually 12V, but this will only cause heating, and unless you're cranking the engine for long periods of time, that heat will dissipate and cause no problems.

    There are other options, such as using a regulator to drop the voltage down, but the automotive electrical environment is pretty harsh and I'd prefer to do everything with relays if possible.
  10. stoggy


    Oct 20, 2013
    Ok so amperage on opening is just under 1.8 and closing is .7 tops. Once the motor stops it peaks slightly on both occasions. When the motor is at rest in open or close when you reapply the trigger (ie, after the limit switches have stopped the motor) it peaks amperage to about 1.1 then back to zero.

    I'm confident the way you described with the two relays I have just swapping the positions on one and using a starter cut out relay will work. Voltage on the system during cranking is still 12.4 volts ...probably due to my beefy battery.

    I will test it out and let you know. If all works I'll install it and post some pics or a video.
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