One SPDT Switch to Drive Relay and Lamp?

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Phrazemaster

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Oct 2, 2006
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Hi, I'm adding an illuminated switch to a green pre, and I was going to use a dpdt switch to drive a relay and an incandescent lamp: one pole of the switch for the lamp, one for the relay. Easy, right?

So I ran across another post somewhere where the person said, why not drive the lamp and relay with the same circuit; if you can drive the relay coil then you also drive the lamp too.

That means I could just use a SPST switch (saving money) and also simplify the design (2 perks!).

So, is this wise? If doable, then would I wire the lamp and Relay in series or parallel, and do I need to worry about coil kickback damaging the lamp?

I'm sure this is tepidly basic electronics, but you know what? I'm a total newbie. Not sure how to proceed.

I was thinking then I should get a 5v incandescent lamp and a 5v relay (makes sense, right?) to drive them both at the same time. I'm assuming I would need to put a diode across the relay contacts, but again how would this interface with the lamp? Anyone willing to give advice/draw a small schematic?

Thanks in advance for the help.

Mike
 
parallel will be fine
series would probably require a resistor in parallel w// relay coil, plus if lamp burns open all is lost.
mind voltage ratings of lamp and relay --maybe try 12v incandescent, won't be as bright at 5v but less current
 
Why do you insist on incandescent? They have limited life expectancy and are a pita to replace.
LED's are almost eternal and draw much less current. With 5 volt you would need a 470R in series.
Having a bulb in parallels with the relay solves the problem of absorbing coil kickback. With an LED, not so; I would recomment a reverse diode (1N400x) across the coil.
Using an SPST makes more sense since the status of the light is a direct indicator of the coil being energized. With a defective DPDT, you could well have a lighted bulb and a relay not turned on (or vice-versa).
 
Excellent points; thank-you shabtek and abbey road.

The main reason for incandescent is price: $3/bulb incandescent vs $11/bulb for LEDs! It's because I picked a cool looking switch that has a proprietary LED. It's also because I want a white light, and they have colored LEDs for that switch for much cheaper, but the white ones are the expensive ones (go figure!). And I have 8 switches that will need lights. That's a lotta moolah for bling!

I can buy 50 incandescent bulbs for the price of 8 LEDs...50 bulbs rated supposedly at 7000 hrs each--so lots of spares.

Now, maybe I should get the switch and try to find a way to substitute a cheaper LED for the fancy one that you're "supposed" to get with it (this is DIY, right?). In general is it doable to put other LEDs in switches, if they aren't made for that switch? I suppose it would just come down to the fit.

I appreciate the wisdom about knowing the coil is energized if the LED is on. If they are in series then if the LED doesn't go on I can know the coil isn't energized. So it would make more sense to put them in series I think... I get the concept if the lamp goes, then the circuit is open...I actually like that; then the lamp is telling me the status of the relay, the lamp, and the circuit as well.

I will see what I can do about finding an LED, but it may just come to cost. It's an NKK switch that has a certain look/feel for the pre I'm building.

Plus there's just something cool about the dimmer more retro look of incandescents...even if they are a pita.

Thanks again,

Mike
 
Phrazemaster said:
The main reason for incandescent is price: $3/bulb incandescent vs $11/bulb for LEDs! ...
mouser price in singles $3.99/incandescent 5V bulb vs $1.79/white 3.1mm LED + current limiting resistor when you cut the LED leads to length yourself.
You might be missing the price for needed beefed up power supply for 8 * 115mA current consumtion/incandescent 5V bulb vs. <20mA/LED as well.
 
Yes, good point, thank-you Harpo. Since I hadn't ordered the switches yet (still planning), I was nervous about committing to it and then finding out I couldn't put in my own LEDs. But as you say, probably I can just cut to fit. And certainly the power supply issue is a point. I have 4 mic pres in the box and 1 torroid xfrmr with 15v outputs, so this is a definite concern.

Thanks all for the ideas; I appreciate it.
 
make your power supply remote.
in its own cjhassis.
adopt a standard power cjonnector.
10 pin twist lock from china for $12 a set.
As the afflicjtion sets in and progresses you will be able to build bigger power supplies/intercjhange them.
ebay ammo box and 10-pins.
not bowling--
they are twist locjk military style, can be disassembled to swap gender from panel mount/cjable mount
over
 
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