Basic Transistor Project Blues

Discussion in 'General Electronics Chat' started by ztrain727, Jun 4, 2011.

  1. ztrain727

    Thread Starter New Member

    May 27, 2011
    Hi Everyone!

    I've been struggling with some time now building what I thought would be a very simple project involving an NPN transistor as a switch. It turned out my total lack of electronic knowledge has made the project quite a challenge.

    So here we go:

    Background: We have a very long driveway that is curvy and lined with thick brush and trees, meaning we have nearly had many head on collisions to date. I want to make a basic remote traffic light system. Initially I had planned to use a motion sensor, but the number of animals wandering around and difficulty of the project discouraged me. Instead, I found some old walkie talkies and opened one up. I attached the speaker leads to a 2n2222 transistor, that is then attached to a 9v battery which powers a light (and possibly a buzzer too in the future). Both our cars will have a walkie talkie on a specific channel and can signal before driving down the driveway.

    Circuit Diagram:

    Well, I don't really know. It's sort of working, the light is just very weak compared to when I directly attach it to the 9v. I tried to measure the voltage but I guess I don't know how to use a voltmeter because the voltage was going down when I triggered the walkie talkie.

    Anyway, I'll appreciate any help I can get. Thanks!

    P.S. I have some other transistors and a small 12VDC 1A relay too....
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    I think this is unlikely to work reliably. How can you be sure that other people will not be using the same radio channel? For that matter, how is this thing to be powered - I would not think that batteries are a good idea, as a flat battery would prevent your safety system working. It all gets easier if the light is mains powered (or, more safely, run from a low voltage obtained from a transformer in your house). Then you would need to have a cable to link it up, which could also carry control signals from your house, and perhaps from a switch mounted at the gate on to the highway.

    Nevertheless, you probably want to know why your transistor does not turn on well from the speaker signal. The chief problem is that it is AC, so the transistor is only switched on when the signal goes sufficiently positive, which will only be part of the time.

    Another thing to notice is that you have no resistance in series with the base, so that there is some risk of excessive current into the transistor, possibly damaging it and the radio as well. You should add a resistance of perhaps 1kΩ in series with the base - this may make the switching action a bit weaker, but at least it would be safe.

    The falling voltage you have seen with the signal on - I assume at the collector - would be normal. When the transistor is off, naturally all the voltage appears across it. That said, your arrangement of returning the collector directly to battery positive with the load in the negative return from the battery to the emitter is unusual - why is that done?

    Perhaps the simplest way forward would be to use the 2N2222 (with a series base resistor for safety) to detect the signal. The 2N2222 could then be given a large collector load resistance returned to +Vcc, with a small capacitor to common to smooth the output. The resulting level could then be used to switch another device, like maybe a PNP transistor to drive your relay.
  3. Dyslexicbloke

    Active Member

    Sep 4, 2010
    Have a look on the web for VOX (Voice operated switch) you will find loads of simple circuits. You could also use a bridge rectifier, connected to the speaker terminals, and an opto coupled transistor so that the light/ buzzer circuit was isolated from the radio. Assuming that the optocoupler output was sufficiant to handle the indicator current you would only need a current limmiting resistor and smoothing cap on the input diode.

    Just a couple of thoughts
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    I thought of rectifying the speaker output, but depending on how much level was present at the speaker this might or might not be successful. Some small portable radios don't put much power into the speaker. A bridge working into an opto would need upwards of about 2.5V peak, and remember that the OP is trying to light a "traffic light" which I would assume is heavier than say a 20mA panel LED.
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010