So I've been getting my head around this circuit. And assuming my learnings are correct, it's been a really good learning experience!
OK – so this is how I understand it.
The fundamentals of the the circuit are the footswitch, the transistor and the relay.
Relay firstSimply the DPDT part is just like any other DPDT switch, the difference here is that rather than being switched by hand it is being switched by an electro-magnet. The electro-magnet in this case is the coil. Once the magnet is strong enough it 'pulls' the switch from its normal state to a switched state. In momentary relays once the electromagnet has no power the switch returns back to its normal state. In our case its normal state is hardwired bypass (In -> Out), our switched state is In->Send->FX->Return->Out.
Simple so far.
The power of the magnet is due to the current through it. As the coil has a fixed resistance (the exact resistance value varies from manufacturer to manufacturer, and relay model to relay model) the coil voltage can be determined to give the correct current through it. There are tolerances given in the spec sheet, such as the minimum voltage (and therefore current through the coil) to actuate the switch and maximum voltage (and therefore current through the coil) before it's switching-mojo is released...... The example I'm looking at gives a switching minimum of 75% voltage (and therefore... meh you get the idea) and a max of 150%. So the coil will be strong enough when 3.75v (=5v[coil rating]*0.75) is applied and will fail when 7.5v is applied (5v[coil rating]*1.50).
Still with me?
In this circuit the relay also has some protection across the coil, this is to stop voltage spikes when switching-off ruining the party.
I am not entirely sure if all relays share the same pinout. Check the spec sheet.
Here you can see the coil 'element' on the left, with the DPDT contacts on the right. Please note the wiper is shown in its normal state. In this case the pole is the left-column of the block of six, the normal is the middle and the switched is the right.
![Image](http://img651.imageshack.us/img651/3484/relay.png)
Next up –
The transistorThis is new ground to me, and makes me want to learn more about transistors. I think I'll have to read the Stompboxology article about this stuff!
Right, in this circuit the transistor is being used, like a power switch. The main principle being used is the saturation of the transistor. Look at the specs of the 2n3904, it gives a saturation voltage for the collector-emitter and base-emitter. At these voltages the transistor saturates, a useful feature of which is that the voltage across the collector and base is 0. If there is 9v at the emitter it will pass to the collector.
Ok so what? Well that brings me to
the switch.
In this case the switch is completing a circuit from V+ to the base, a simple operation hence why a simple switch can be used. Please note that it must be latching as the relay is not. The completion of this circuit passes V+(ish) to the base causing the transistor to saturate, thus passing the voltage at the emitter to the collector (+9v -hurrah!).
Please note, the resistor in front of the base is to limit current. Without it the base would overheat and eventually fail.
So now we know what the switch is doing and that it causes the transistor to switch. What happens after that.
In this circuit the V+ serves two purposes.
First - You will all be familiar with the LED current limiting resistor and the LED. Right. Well when the switch is pressed, the transistor saturates, V+ then passes through, the LED resistor limits the current and the LED lights up. Yay. Indicator everywhere!
Second – The voltage is also applied to the coil in the relay (remember that? This turns on the electro-magnet and is 'pulls' the relay from normal to switched. In our case from bypass to FX.
The happy-happy-joy-joy bit is that they happen at the same time, and because our switch is latching it stays like this until we unlatch the switch. Nice!
OK – at this point if you have a 9v-coil relay you're sorted. In fact if you have a 12v-coil relay you're probably sorted too (as a quick justification, taking the earlier example of a minimum switching voltage of 75% - 12v*0.75=9v, therefore the switch will actuate. Unless you're using knackered batteries...).
If like me you're desperate to find ANY solution we are not out of luck. We can use a 5v-coil relay by using a series resistor to drop the voltage across the coil. The voltage passed by transistor is about 9v, this is supplied to the LED and coil in parallel. We don't want to touch the LED supply so we place the resistor between the LED resistor and the coil. This is placed in series.
Now stick with me here. My maths is pretty ropey (as may well be most of the above!).
We use the equation V=IR where V is the voltage, I is the current and R is the total resistance.
In this example V = 9v (collector supplied), I is to be found out and R is the resistance of the coil plus any additional resistance. Lets guess at a resistance bigger than the the coil resistance (so 210ohm)
So: 9 = I x 70[coil resistance]+210[limiting resistance]
Rearrange it to: 9/(70+210) = I
or more neatly I = 9/280 = 0.0321A
We can then use the V=IR again to work out the voltage across each element:
Voltage across limiting resistor: V = 0.0321A x 210 = 6.741v
Voltage across coil: V = 0.0321 x 70 = 2.247v
As a mathematical exercise: Voltage across limiting resistor + voltage across coil = 6.741+2.247= 8.988v
Not exactly 9v as I rounded the current value but you get the point!
OK. Looking at those we can see that the voltage across the coil is too low to actuate a 5v(75%) relay: 5v*0.75 = 3.75v the voltage calculated is 2.247v. No dice.
Looking at V=IR where I is constant, if we decrease R – V follows suit. We want less voltage dropped across the limiting resistor so there is more left for the coil.
I've done a couple of calcs to find it out but the resistor we need to get 5v across the coil is... 56R:
I = 9/(70[coil resistance]+56[limiting resistance])
I = 9/126 = 0.071
Voltage across coil = V = 0.071*70 = 4.97v
Tahdah!
OK. So there we have it. I will endeavour to test this as soon as possible. Probably should have done that instead of writing this!
Anyways, I hope it is all correct – PLEASE CORRECT ME IF I'M WRONG AS I NEED TO KNOW! - and I hope someone gets to learn something as I have.
Peace, love etc etc.
AND IM SPENT.
Ian.
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