Alternating Flashing Lights Circuit: Measured voltage oscillates, but bulb does not flash.

Discussion in 'The Projects Forum' started by Wayne023, Dec 31, 2015.

  1. Wayne023

    Thread Starter New Member

    Dec 31, 2015
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    I built a circuit on a breadboard to alternately flash two lights. This is the first circuit I have ever built using resistors, capacitors, transistors, and a diode. I intended to use it to flash a crossing signal on my model trains. I used a circuit designed by M Rollins (attached and available at www.mrollins.com/flash3.html). When I check the output voltage with my AC voltmeter, I get four repeating readings of 12, .1, 6, and 0 volts. When I connect my crossing light to the output, it lights and stays lit. It dose not flash. Can you help me understand this? Where the circuit schematic says output 1, I connected a red and black wire which I connect across the voltmeter or the light bulb. I did the same for output two. Was this the correct interpretation of the outputs? Both outputs result in the same voltmeter and light bulb results.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,693
    This can flash one light, or two lights alternately. Do not connect one light across the two outputs. Connect to an output and ground. If you don't need two, (just want one flashing light, just ignore one output.

    Before connecting, let us know the type of light you plan to flash. Usually, the bulbs are connected between the low value resistor and the transistor. Then you save some energy and everything runs cooler.

    PS: welcome to the site.
     
  3. Wayne023

    Thread Starter New Member

    Dec 31, 2015
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    Thank you. I plan to light bulbs marked 53R 14.4V. They are red bayonet style 14V bulbs.

    I take it from your response that I should have made the connection across the word "output" in the circuit diagram and connected one side of the bulb to that wire and the other side to ground. Is that correct? What I actually did was connect the bulb across the word "output" in the diagram with the bulb being the only connection between the red and yellow wires on the diagram. With no bulbs connected, the circuit is open in both places across the words "output". Should I make those connections and then connect one side of the bulb at this closed connection? The bulb would then be connected between the red wire just before the word "output" and ground.
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
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    No, that's not the way to connect them. You had it right.

    First, you need a DC voltmeter. An analog movement would be good for this app.

    Turn switch off and measure DC voltage at input to switch to ground.

    Install a 500 ohm 1/2 resistor at each output.

    Turn switch on and measure voltage at switch input again. What is the difference in voltage?

    If your voltage didn't drop off too much, You should be able to see voltage ramps at the junction of the 500 and 22 ohm resistors with analog movement.

    If you do, replace 500 resistors with the bulbs. Check the voltage at the switch again.
     
  5. Wayne023

    Thread Starter New Member

    Dec 31, 2015
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    Thank you. I was confused, so it is good to know that my interpretation of how to connect the bulbs was correct. My digital meter will measure DC volts. I'll see if my old analog meter still works, and I'll try what you suggested. It will take a few days, though, since I will need to get the 500 ohm resistors, and that will probably not be possible until 2 January. I'll get them and make the measurements you have suggested. Happy New Year!
     
  6. BR-549

    Well-Known Member

    Sep 22, 2013
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    The 500 ohm resistors was to partially load the power supply. Put your bulbs in and measure voltage at switch at on and off.

    What's the voltages? DC.
     
  7. Wayne023

    Thread Starter New Member

    Dec 31, 2015
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    The DC voltage at the switch input with the switch open was 26.7V. With both bulbs in the circuit and the switch closed, the DC voltage at the switch input was 23.9 V. I discovered that with both bulbs in the circuit, the bulbs flashed alternately as advertized. With only one bulb in the circuit and the other output open, the bulb was on steadily. Thank you for helping me. The circuit now works as expected. Happy New Year!
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    782
    Filament bulbs have a positive temperature coefficient, so the low cold resistance could be too much for the transistors to switch fully on.

    With decent rated power transistors, you may just need to reduce the base resistors and scale the capacitors accordingly.

    There are various options, like using Darlington transistors, you could pre-bias the filaments with bypass resistors so they're just barely glowing - but hot enough to increase their resistance - or just use LEDs.
     
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