My "Building a basic Darkroom" advice needed thread

Chan Tran said:

Driving a relay is easy. You should consider driving 2 relays instead of 1. One is electro-mechanical type and the other solid state type. Some enlargers light source don't like solid state type but otherwise the solid state would last longer. Relay specs of 120VAC 10A should be about right. If you want to control a specific enlarger then you get one that match it better.

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So, I assume that the solid state relays change the original waveform from sinusoid to something else, maybe square or something simiral right? If so, incadescent lamps wouldn't have problems. Am I right?

Chan Tran said:

I wouldn't use the 555 chip with RC circuit as it's not very accurate. Building from scratch I would build a counter that count the 60/50hz power line frequency.

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Taking into account that RC components have tolerances that will affect the produced frequencies, you are of course right. 5% tolerance is quite typical and all of them combined could give you quite different results. Even worse is the case where the tolerance of each componet is not added, but multiplied. OTOH, accuracy might not be exactly what you need. Repeatability certainly is and I think it's granted.

repeatability is correct!

Just another question, considering that my enlarger is 12V and draws around 6 to 7 amps (the sticker on the side states 75W max rating for the halogen globe), would I be cutting myself short if I stuck with a 12Volt, 10 amp relay? Should I gear up for 110V max (do many enlargers run @ 240V?), which will obviously add to the cost & possibly make an off the shelf item in the long run a better proposition?

Anon Ymous said:

So, I assume that the solid state relays change the original waveform from sinusoid to something else, maybe square or something simiral right? If so, incadescent lamps wouldn't have problems. Am I right?

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Solid state relays are usually optically coupled so the input is about 3-15 volts dc. The actual 'relay' is either a triac or SCR. For AC mains you need the triac version.

It doesn't convert the waveform, it just switches it on and off. The solid state relay goes in series with the lamp circuit in place of (or as well as) a standard switch.

http://uk.farnell.com/opto-22/120d3/relay-solid-state-3a-120vac/dp/1426694



Steve.

hoffy said:

repeatability is correct!

Just another question, considering that my enlarger is 12V and draws around 6 to 7 amps (the sticker on the side states 75W max rating for the halogen globe), would I be cutting myself short if I stuck with a 12Volt, 10 amp relay? Should I gear up for 110V max (do many enlargers run @ 240V?), which will obviously add to the cost & possibly make an off the shelf item in the long run a better proposition?

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Wait, don't you have 220/240V down there? If so, you'll need a relay for the socket voltage of your region. The timer shouldn't change the voltage. I assume that your enlarger has a built-in transformer, which feeds the lamp with 12V. Better tell us the model of your enlarger. Someone might be able to give you the specs needed. BTW, the 75W max is for the lamp alone. The enlarger as a whole is a different load > 75W.

Steve: Thanks for the clarification.

I realised this as I went to bed last night. No, my enlarger doesn't have a built in PSU. The PSU is an external Fixed Voltage (12V @ 10amp) regulated PSU. I should have realised that this is the exception, not the norm! I was going to cut the power between the PSU and the enlarger (with my othe hobby, slot cars, we only deal with the low voltage DC side of the circuit. Much easier, much safer). Obviously, this will restrict this Timer to the enlarger that I am using. New enlarger, new timer!

The only thing I haven't checked with this setup is whether when the power is cut to the PSU, does it automatically cut the low voltage side. The 30amp PSU that I am using on my slotcar track takes a few minutes (with no draw) for the caps to drain. Obviously, this is going to be a pain! (I get the feeling that the previous owner used the safe filter). If this is the way it is, then I will build as quickly and cheaply as I can, a timer circuit.

But, if the PSU works as I would like, its probably more convenient and probably cheaper (in the long run) to get an off the shelf unit, even if its a basic one (I have seen a Novex R on our favourite auction site relatively cheap.)

Well, Christmas has come a little early again today! To help out with the project, I received an old Smiths Analogue mechanical timer, a masking easel and an oldish Patersons Safe light.

In relation to the safe light, this is the older Patersons style, which is like a tapered tube (as opposed to the newer ones that look like a mushroom). Does anyone have any opinion on whether these would be OK? I noticed that the lamp inside is a Red lamp as well. I thought that these can take a standard lamp. Am i wrong? I do plan to do a fog test on the paper (when I get there!), but if its not worth my hassle, I will look for an alternative.

Cheers