# Help with understanding a circuit

1. Dec 13, 2011

### mhache

1. The problem statement, all variables and given/known data

Hi there,

I'm fairly new to electronics and am hoping to learn how the attached circuit works. It is from a Kip Kay project over at Make Magazine (http://blog.makezine.com/archive/2008/10/weekend-project-ultimate.html)

I have posted the question over at the site but the project is over 3 years old so will not likely be answered.

The circuit drives (via relay) two sets of led "words".

The relay is not specified but I'm assuming it's a 9V SPDT.

What I'm trying to understand is the capacitor/transistor/relay arrangement. Both transistors have a 330uf cap attached with one lead to the base and one to the collector. The collector of each transistor goes to one side of the relay coil. One of the transistors has a resistor between the collector and the relay coil (the one going to the more positive side of the relay judging by the diode)

2. Relevant equations

I'm hoping someone can explain how this arrangement alternatively provides current to the relay and then not.

Thanks

Marc
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### flashing led sign circuit.jpg
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2. Dec 13, 2011

### technician

This may not be much help!!!! Are you sure your circuit diagram is correct?
It looks to me as though it is supposed to be a MULTIVIBRATOR using 2 transistors..... this would switch the relay on and off repeatedly.

If it is supposed to be a multivibrator the capacitors should go from base of one transistor to collector of the other transistor.... they look like they 'cross over' in a circuit diagram. The diode across the relay coil is to protect transistors when the relay is switched off.... it does not influence the operation of the circuit.
I am not really sure what the circuit and relay are supposed to do so this is very limited help but at least you have something to think about.Hope it helps and get in touch if you need more help.

3. Dec 13, 2011

### Xitami

click

4. Dec 13, 2011

### Staff: Mentor

...and for the OP: in this circuit R4 would be replaced by the relay coil with its transient-suppressing diode in parallel with it.

5. Dec 13, 2011

### mhache

Thank you both. I kind of figured it was supposed to be a multivibrator. What I'm still trying to wrap my head around is how that works with a relay. I've seen multivibrator circuits with a load between collector and ground (eg in the venerable Radio Shack Getting Started in Electronics) and as I understood it the capacitor drives the charge alternatively between the two loads.

So if a SPDT relay connects one circuit when the relay is on (coil energized) and another when relay off, then am I correct in this case in that one of the capacitors provides the charge to activate the relay (in the original diagram, the one with the resistor (connected to the most positive terminal of the relay)) and the other capacitor discharges to ground (most negative part of the relay)?

Lastly, if the above is correct, how do I determine how much current is delivered to the relay ? It is simply Vsource (9v in this case)/resistor or do I have to account for the voltage drop across one or more of the transistors ? Completely off base ?

Since the relay model isn't specified I need to figure out any adjustments to deliver the proper coil current.

Thanks so much. I was really scratching my head on this one. As I'm still new to this I wanted to make sure I understand whats going on.

Marc

6. Dec 13, 2011

### Staff: Mentor

Here's what your circuit should probably look like. The one you found on the project's website was a travesty.

A description of how the circuit functions can be found here.

It's transistor Q2 that provides the conduction path for the relay to operate. Q2 behaves as a switch. The capacitors serve as timing elements in conjunction with the base resistors. See the website linked-to above.

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• ###### Fig1.gif
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7. Dec 13, 2011

### mhache

Thanks again everyone. The fog is starting to lift. After reading the wiki and The link from gneill I thinking understand the concepts. If I could beg your indulgence for a follow on question.

In the diagrams below r2 and r3 act with c1 and c2 to acts as timing elements as gneill calls it. In the application I am discussing there is only one load (the relay) so what role does r1 play and how do you determine it's value ?

Thanks
Marc

8. Dec 13, 2011

### Staff: Mentor

R1 acts as the "load" for Q1. Without R1, Q1 couldn't conduct. A reasonable way to choose its value would be to have it match (approximately) the coil resistance of the relay. That would help make the circuit symmetrical in operation, keeping the timing equal for both switching states.

I don't know where you're planning on buying your parts for this project, but if you can find a suitable relay look for its data sheet (specifications) which should mention its coil resistance. A quick search for a (gasp!) Radio Shack component turned up a 9V relay, circuit board mount, SPDT, with a 500 Ohm coil resistance. So the 560 Ohm resistor in the original circuit is probably a fair choice.

9. Dec 14, 2011

### mhache

Again, thanks for the help. What i'm hoping to bulld is a fan sign for sporting events. The vibrator circuit (above) will flash the words (made up of one or more LEDs) alternatively.

Not sure of the protocol here, if I should start a new topic, please let me know if I should.

I was doing some more reading as what I would really like to do is have the words flash in an ABA order ie "Go Jets Go", small pause, repeat. I'm thinking of using a 4017 decade counter, triggered by a 555 in astable mode. Counts 1&3 would light the first word, count 2 the second word and count 4 would have a load resistor to provide the pause. Count 5 reset.

Does this sound like a good approach or is there a simpler way? If you have time let me know your thoughts and I'll take stab at a circuit.

Thanks
Marc

Last edited: Dec 14, 2011