Homework Help: Simple Circuit

i would help, but can't open the attachments. They are not working.

 

What is the circuit meant to be and for what purpose???

The Bob (2004 ©)

 

Where is the circuit? The attachments won't open.

 

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 ©)

 

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?
:rofl:
+af

 

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.

 

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......