1.5 pulsing DC to 12DC how?

Discussion in 'Homework Help' started by 5&under=sorcery, Jul 4, 2010.

  1. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    My neighbor's electric gate quit. The control section of the board is wiped out. The power side is fine. I have already figured out how to repace all the defunct electronic controls with a mechanically held relay which can perform all the functions that were previously done electronically via 4 ordinary 12 volt magnetic control relays. The only problem is the signal from the receiver which is activated by the hand held one-button transmitter. It works thusly: There is a constant 3.5 volts DC on the circuit. When the transmitter button is depressed the voltage drops to 1.5 and pulses which I am guessing is square wave DC. I don't have a scope. Then upon button release the volts drop to zero and rise again to continuous 3.5. The continuous 3.5 is useless because it is there all the time. I need to either transform or amplify the 1.5 pulsing to 12 volts AC or preferably DC, or utilise the drop to zero and back to 3.5 to end up with a 12 volt one-time pulse. I know a mosfet would work if I had a recognisable 5 volt or so signal but I don't. I have continuous 5 and 12 volt DC to work with outside of the signal circuit.
     
    Last edited: Jul 4, 2010
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    1.5 should be easy to work with. There are thousands of 'AA' battery circuits out there.

    As for the pulsing, is the power supply in this unit a switcher? If so, you may have an osc problem. Under load, if you lost one of the caps on the osc, it would slow the switching considerably.

    I would check that.

    Check all the caps. Whatever is supposed to be smoothing this signal, is not working. ;)
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    What model number is this electric gate opener?
     
  4. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    It's a Mighty Mule 1000. The circuit in question is a product of what is taking place inside the 3-wire receiver when the transmitter is activated (button pushed) and then de-activated (button released). It's probably an extremely common circuit that is reproduced by the tens of thousands for gates, garage doors etc, but none of the companies that sell this stuff will answer my question.
    The transmitter is powered by 5V DC on the red and black wires, the third green wire simply runs to ground through a 2.2 Kilo ohm resistor, so I'm thinking the original programable IC must have been able to recognise either the pulsing or the voltage drop to zero and return to 3.5 volts when the button is released. Heck I don't know, a nice straight forward 5 volt return I could deal with but but I'm a power electrician. I'm still not even sure what that signal is and the receiver is sealed so I can't get into it and destroy.......I mean look at it.
    By the way, the motor and micro switches in the ram are fine, the ram operates as designed when simply powered from a battery.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Can I assume you've visited http://support.gtoinc.com/Support/TroubleShooter.aspx and look at the Mighty Mule 1000 information pertinent to the problem?

    I hate to give you the bad news, but when you measure the black and green wire from the receiver, it is suppose to be 3.75 volts and when you press the button it drops to zero.

    Visit that website.
     
    Last edited: Jul 5, 2010
  6. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    I have spent hours there. It's not bad but it doesn't get into the depth at which my problem lies. The only thing their troubleshooting results in is "replace the part". You can't blame them and I don't. They aren't in business to help people who don't understand electronic componentry and circuits. Once I have a transmitter circuit signal I can translate into 12 volts AC or DC I'm in business. It's this transmitter/receiver circuit that has me stumped. Heck, a whole new board is 170, but what would I learn by just buying a new board?
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    3 volts DC would measure as 1.5 Volts DC when changed to a 50% Square Wave of 3 Volts Peak.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    have you manually operated the gate by jumpering the appropriate accessory pins?
     
  9. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    Yes. Everything works fine on the power side. The ram functions fine. I need to be able to translate the return signal from the receiver into something I can use to drive a relay or mosfet.
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The first thing to learn is ensure you do all the recommended tests before attempting a redesign.

    You could draw a schematic on the control board to gain an understanding of what they are doing.

    Can't you purchase the components from an electronic store, there are many. Is there a proprietary chip on the board?

    Alot of the "replace the part" concept comes from the cost to repair the item, time and labor. There is an opportunity for someone to repair those parts and sell them as refurbished.
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    So if you also replicated how the receiver signal acts ... 3.75V to zero and back to 3.75V ... the gate opens and closes?

    Can you post a close up picture of the control board and the receiver board?
     
  12. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    The programmable integrated circuit which is the brain of the control section was burned out. You can't buy another one. You can buy the chip but it won't have the programming in it. It'e re-design or replace. Re-design is more fun.
     
  13. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Use a Capacitor to pass the AC from the Pulses and block the DC.

    Filter and Rectifiers used on the pulsing DC will recreate DC. Apply that to the base of an NPN. Use that NPN through a resistor to drive a PNP on the 12 volt line with a big capacitor and your relay in parallel between that PNP and ground.

    The trick is only in blocking the normal DC signal from turning on the transistor which prevents you from applying the signal directly.

    I acually simmed a circuit that might work but we don't know what type of pulsing signal you have to work with. I don't know enough to say how close it is.
     
  14. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    The capacitor does nothing. I'm fairly convinced at this point the signal is some kind of electronically created DC. What I did find is that by measuring the green against the red positive instead of the black negative, the signal increases instead of decreases. It goes up to 3.75 pulsing and then jumps to 5, gotta get my digital out to be sure exactly how much, but it hangs at very close to 5 for about a second before it drops back to 1 1/2. Maybe I have something to work with here after all. I can polarise a relay coil either direction so this might work. Will look into a mosfet now too.
     
  15. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    A Capacitor alone would do nothing. I didn't expect it to anything.
    I didn't want to post until we knew more, because there is more information needed before we can see what changes are needed to make this match your situation.

    [​IMG]
     
  16. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    Gulp. As a power electrician I recognise some of the components but the total package is way over my head. When I have an old fused panel torn out and there are wires sticking out everywhere people ask me how I can possibly put all that back together. I tell them it's really quite simple once you know what is going on. Looking at your diagram I know how they feel LOL.
     
  17. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    It is a very basic High Pass with a Schottky rectifier.

    It just passes the AC then turns it into DC again. That will flip the transistor switches, with the output under the PNP having a Capacitor to catch any pulsing and keep the relay from being buzzed in case the pulsing sneaks through.

    And I should have mentioned that the R4 1kOhm is actually meant to be the relay. That test point looks like it is meant to be a feed to the relay but it was just a test point for the voltage on top of the Relay.

    If your relay has less than 1kiloohm which is likely I suggest you use a heavier PNP like a TIP125. That is a Darlington type but for a lower gain power PNP you would also want to decrease the bias resistor on the PNP base to about 12k.

    I am still very uncertain about this because I have not seen your pulsing DC.
    It all depends on that. I have guessed that it is actually a 3VDC at 50%.
    I also gave it a reasonably low Frequency. Higher frequency this should work fine. +-1.5V it should still work. +1.5 DC it should even work, which is one of the reasons I specified a Schottky Diode.

    So I think it is a good guess at what will work but without looking at a scope of that input it would need to be a lucky guess.
     
  18. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    A scope WOULD be helpful wouldn't it. Given that I can actually watch the needle on a VOM flutter I agree it must be relatively low frequency. I'm pleased to see that rise to 5 volts when measured against the positive side and the 1 second or so hang time there. That should be plenty of time to actuate a device. I would still love to know how that circuit works in it's original configuration.
     
  19. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    This circuit is not a latch. It will only hold the relay on for as long as it is getting a signal. If you wanted to run a 220k resistor from the collector of the PNP to the base of the NPN then it would be a transistor latch.

    You would need some way to turn it off like a limit switch that opens when the job is done and shuts down the circuit power.

    If the limit switch suits part of the plans anyways then making the cicuit a latch will actually make it better able to tolerate a lower frequency that is tricky to squeeze through the capacitor.
     
    Last edited: Jul 6, 2010
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