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Homework Help: Simple Circuit

  1. Jan 24, 2005 #1
    Hi I'm trying to build a simple circuit but my physics is shaky. Could someone be kind enough to take a look at what I've done and notice any rookie mistakes? Thanks! The top side is positive.

    Attached Files:

  2. jcsd
  3. Jan 24, 2005 #2
    i would help, but can't open the attachments. They are not working.
  4. Jan 24, 2005 #3
    What is the circuit meant to be and for what purpose???

    The Bob (2004 ©)
  5. Jan 24, 2005 #4
    This is trigger for a camera flash unit. It's suppose to fire my flash unit when the infrared sensor gets interrupted. I don't know if I'm building it right. In any case nothing fired.
  6. Jan 24, 2005 #5
    Where is the circuit? The attachments won't open.
  7. Jan 24, 2005 #6
  8. Jan 25, 2005 #7
    What time delay are you looking for between using the unit and it flashing??? (I ask because you are using a 555 timer circuit)

    Sorry but this circuit layout is not familiar to me (some of the symbols do not look like the ones I use). Is the transitor connected to Pin 8 of the IC??? (Unless it is not a transistor and your use of symbols is different to the ones I use)

    The Bob (2004 ©)
  9. Jan 25, 2005 #8
    To have an adjustable delay would be nice but I don't think that's part of the design. If it fired instantly would be acceptable. Pin 8 on the diagram is connected to the positive from what I can tell. The phototransistor (infrared detector) is diagrammed to be connected to pin6 i think. Thanks for looking.
    Last edited: Jan 25, 2005
  10. Jan 25, 2005 #9
    how things work....

    i'll have to review the connections of the 555 timer, but the gist of this circuit is that while the beam from the LED shines on the phototransistor, the phototransistor acts like a lower resistance than when there's no light hitting it. the phototransistor and the variable resistor (the 10k with the arrow through it) form a voltage divider; light shining on the PT makes pin 6 of the 555 go up in voltage; cutting off the light makes that voltage drop.

    this drop in voltage is what's supposed to trigger the 555.

    the desired result appears to be that cutting off the light from the LED to the PR will cause the 555's output on pin 3 to suddenly go up (increase voltage), sending current into the gate of the SRC.

    the SCR would probably be connected (the two "output" arrow lines) to the input or remote trigger socket of your strobe, and by that i infer you mean "photoflash" unit, because those two connections alone won't fire a strobe lamp alone... many more components are needed to do that! :)

    here are some things to test for, and if you're going to experiment with circuits like this, you should really try to have some kind of simple Volt-Ohm-Milliammeter handy, or at least volt-ohmmeter.

    with light shining from the LED to the PhotoTransistor, measure the voltage from pin 6 of the 555 to ground. cut off the light between the LED and the PT and see if the voltage changes. if it doesn't, the adjustable resistor may be set to too low of a resistance, or the LED or PT may be defective.

    if that seems to be working right, interrupt the light again and see if there's a short jump in the voltage from pin 3 of the 555 to ground.

    if it's too short of a pulse to see on the meter, it gets trickier. a short pulse may only be visible if you can hook an oscilloscope to those pins and trigger the 'scope off pin 6...

    next, the SCR may be defective, or it may not be getting enough current through the 10k fixed resistor between pin 3 and the SCR's gate connection. trying a 5k resistor may help, or again, look at the SCR's gate with the 'scope, too. you should see a pulse at the gate, but it will not be much more than .5-.8 volts or so, while the pulse at pin 6 could be much higher.

    finally, your strobe may not be the kind that is triggered by this kind of circuit. this circuit will trigger a photoflash that's set off by shorting the input pins (that's what the SCR does in this circuit, effectively.) or, ...... well, let me know if any of these suggestions work, first....

    and the 555 may be defective, too. like most integrated circuits, it should not be exposed to electrostatic discharges, like you might get if you were holding it as you walked across a carpeted room in a low-humidity environment, and then touched IT to its socket before you touched YOURSELF to the ground side of the circuit.!

    (edit:) i checked a similar 555 circuit i've got, and it looks like your connections to pins 1, 3, 4, 8, look ok; i'm not sure about 2, 5, 6, 7. (/edit)

    ain't electronics fun?
    Last edited: Jan 25, 2005
  11. Jan 26, 2005 #10
    Thanks a lot for taking a look at the circuit. Your post was very helpful. I thought it a good idea to test different areas of the circuit to see they worked but did not know how to nor what to look for. Your post outlined it exactly thanks.

    that's right I have an external flash that has an adapter to trigger it. I output wires from the scr to the adapter.

    For now I'm using a sepearte tv remote control as the LED. I don't know what it means to connect to ground. When I connect between pin3 and negative I do get a voltage reading of 1.22. Then when I use a seperate remote control and press a button I get around 1.55. The 9v battery is 7.55.

    When I do that I get an initial reading of 6 (remote on) to 6.5(no remote).

    I have no access to an oscilloscope but it seems to be working as per your description.

    between pin3 and the gate on my first attemt i get 6.11 (no remote) 5.55 (with remote).

    now I seem to be reading 5.8 no remote then jumps to 4.7 as i press the remote then it settles on 5.3 as i hold the remote. (this is using the 4.7k resistor as suggested below)

    I've installed a pot instead of the 10k resistor and the circuit now looks like this.

    I don't know if I installed it correctly but the front of the pot looks like this.

    i know that if i disconnect the leads to my flash and then touch them together it will fire.
    Last edited: Jan 26, 2005
  12. Jan 26, 2005 #11
    Success!! I don't which of the solutions did it but after checking those values and some tinkering it now works. Thanks a lot plusaf! :rofl:
    Last edited: Jan 26, 2005
  13. Jan 27, 2005 #12
    ok slight problem which i'm sure there's a simple solution. The circuit works sometimes. What I means is that when it works it will consistently work firing everytime but sometimes if turn off the circuit and come back to it it won't fire at all. Is this related to grounding?? I'll give an example one time it wasn't working so I did some testing with the multimeter. I touched the positive on pin3 and the negative to negative then all of a sudden it started working. Further experimentation shows that sometimes it fires when I take measurements from pin3 to the gate sometimes from pin3 to ground and sometimes pin3 to positive. It keeps moving around. How do I fix this?
    Last edited: Jan 27, 2005
  14. Feb 1, 2005 #13
    dang... where did i put my copy of the circuit???

    "ground" in the circuit, as i recall, is the negative side of the battery....
    the symbol is the collection of three or four parallel lines of decreasing length. it refers to a common connection point for various parts of a circuit, from which, as a reference, all voltages or signals are measured.

    if touching the multimeter from pin 3 to ground "reset" the circuit and made it work again, one problem might be that the scr isn't turning off, or that the 555's voltage at pin 3 doesn't drop enough to let the scr turn off. try putting another resistor from the scr's gate to ground, maybe another 10k, like the resistor from pin 3 to the gate. that might help it shut off, if that's the real problem. actually, the resistor from the gate to ground (the SCR's cathode terminal) could be as low as maybe 5k or so.

    what's the voltage across the battery?
    i'm a little concerned that the output of the 555 (pin 3 that drives the SCR's gate through that resistor, right?) isn't changing enough. i would have thought that the swing would have been more like several volts.

    back to some more research......
    http://home.cogeco.ca/~rpaisley4/LM555.html#4 has a bunch of general info on the 555.... i googled "555 timer circuits"; maybe i should have tried "trigger circuits" instead....

    also... http://www.kpsec.freeuk.com/555timer.htm

    looks like pin 6 is a threshold level, and pin 2 is the trigger input.
    Last edited: Feb 1, 2005
  15. Feb 2, 2005 #14
    Plusaf, thanks a lot for the reply. This thread would be positively dead without your replies. I'm going to test this out as soon as my new rechargeable batteries are ready. I've been killin' so many alkalines. I'll update later.
  16. Feb 3, 2005 #15
    Okay, it was the resistor from pin3 to the SCR that was causing the problems. I ended up taking it out. I wanted to add an LED so I could see when the beam was armed. I just attached it to pin6 though I don't know where the best place would be. Seems to work though. Here's the final layout. I'll be adding a delay circuit to this shortly...

  17. Feb 9, 2005 #16
    Taking the resistor to pin3 was a bad idea. It worked for the half dead 9v I was using. But after I switched to a new one it heated up the timer chip really quickly. I did try adding a resistor to the gate but it didn't seem to solve the problem.
  18. Feb 22, 2005 #17
    getting closer?

    running a wire directly from pin 3 to the gate of the scr will make the output of the 555 think it's driving pretty much a short circuit (the gate can't go much above .6v, no matter what the 555 tries to do), so having some resistor there is a good idea. if you're running about 9 volts into the 555, and for rough numbers, you want to drive, say, 20mA into the gate of the scr, a resistor in the 400-500-ohm range should work fine.

    the scr has to turn off before it can be retriggered, and that can only happen if the current through it drops to near zero or if the circuit it's hooked to tries to reverse the current through the scr. this is pretty normal and isn't a concern to the scr, but if the circuit you're triggering with it doesn't let the scr's current drop to zero, it won't go off, so it can't be triggered ON again (since it's never gone off.) putting another 470-ohm or so resistor across the scr's gate to cathode (grounded side) may help, but it's not a guarantee. the real turning off has to come from the strobe it's triggering. one check here is to disconnect the strobe after every firing to see if that lets it be triggered right "every" next time. or you can put a momentary switch in series with (between) the scr's anode and the strobe, to open the circuit when you press the switch. sort of a reset button, if all else fails.

    onward and .... whatever direction......
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