transistor as a relay driver

Discussion in 'General Electronics Chat' started by yourownfree, Mar 4, 2013.

  1. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    The transistor is a 2N2222A the relay a 5 volt axicom d3009 with 30.1 ma coil draw. How can I take a voltage of .5 volts DC as the data line and be able to turn on relay operated at 5 volts DC ? I can use a darlington as well using two 2N2222A's if need be or whatever else works. Dont I have to overcome the .7 volts to turn on the relay driver transistor to saturate for the relay?
    I don't have to use bipolar I can use anything that is suggested.
     
  2. #12

    Expert

    Nov 30, 2010
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    More than one way to skin this cat. Being an analog guy, the first thing I think of is an op-amp with gain, but a comparator will do it, probably better than an op-amp. Bottom line, you aren't going to do this with a single transistor. Maybe 2 or 3 transistors if you have a negative supply voltage, but an IC is the simplest way. Are you following this, or am I going too fast?
     
  3. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    following perfectly. It just never entered my head. too simple. Thanks will give it a try. I have plenty of op amps and even comparator chips as well. Yeah sounds good. heck I bet even an lm386 audio amp chip would fit the bill nicely. what do you think?
     
  4. #12

    Expert

    Nov 30, 2010
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    I haven't looked at a 386 for years, and I never considered how it would work as a comparator. I think the auto-midpoint output voltage will be a problem.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    0.5V on a data line, is that the low output or the high output? I'm assuming binary on/off since you want to switch a relay.

    If you have a higher voltage as the opposite output, you could simply invert the binary signal to drive a transistor.
     
  6. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    .5 volts was the on state with meter on dc volts. although it is ac as well involved or maybe I dont really know. What I am doing is using a ham radio audio output to key a relay. I had planned on adding a pot to control the input of the driver. that way audio can be set for the proper level for the other radio the audio is going into, then also use the audio to drive the relay driver to key the other radios transmitter. The audio does not come on until the squelch is broke. I quickly stuck my meter on the speaker terminal and measured .5 volts. cant say if that was at full volume or not. So I am assuming the dc component of the output was around .5 volts. The final result will be a repeater for 2 meter ham radio. This might spark some other ideas.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Sounds like what you are building is a Voice Operated Transmitter, or VOX. These circuits remove/automate the "Push To Talk"/PTT requirement.

    Is this correct?
     
  8. #12

    Expert

    Nov 30, 2010
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    Sounds like I answered your question when I didn't know half of what you wanted. Please consider that as you continue.
     
  9. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    Well almost a vox but not. More carrier operated. I will be using the dc portion of it to set it off. Just as soon as the squelch breaks a voltage appears. If i waited for audio i would miss the first part of what they say. I am sure with using a op amp circuit i will be fine. I will build tonight hopefully. I will return with the results and final diagram to share with others. Thanks for the input.
     
  10. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    Ok after I submitted this help question I did not need the circuit anymore as another person did his own thing on the radio. But just because I had to know I had to mess with it. The final result is, it's just not worth it. Easier to use a Darlington circuit. #12 was right about the midpoint as a problem. I spent enough time on this to know I have spent enough and I was defeated, which in this case it really doesnt matter anyway.
    What I did try was a conventional circuit, protection diode across the relay. Relay hooked to source voltage and to pin 6 of the lm386-1(the small one). Made sense to me to do it this way, but it just would no fire. I tried swapping inputs and even used gain of 200. I tried to single out the output transistors in the IC so I was just using one of them at a time but still no go. I was able to fire the relay but the circuit just could not saturate the output transistors. I attribute this to them running at half the voltage of the source as they normally do. The relay I was using was an axcom 5 volt relay. The source voltage I raised up to about 10 volts to give me enough to run the relay, but I just could not make it work. It would turn on the relay just by applying power in one of my configurations but I could not get it to switch off or change states. So semed like a good idea but if I have to add a transistor to make it work I just defeated the whole purpose. I have seen this chip work as a comparator in the hound dog metal detector circuit, I tried to make long ago but even the it was so unstable it was worthless. I really have no need for this kind of circuit at this time so I do appreciate the comments, makes this whole thing fun when you have friends to talk to and share stuff. But I think we are done here. The whole idea of this circuit was to use one chip that had a lot of gain and be able to operate a relay, and to keep the parts count at a bare minimum. I can build other circuits no problem, but this was just a challenge to see if it could be done. What would have worked would be to use an opto-relay. I am sure then it would have worked, might need to use a pull up or down resistor at input of opto to hold it til the comparator kicked it to change the state.
     
    Last edited: Mar 26, 2013
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