Enlarger metronome control

[Joe O'Hara]

Michael A. Smith recommends using a metronome and foot switch to time your printing operations. The idea is that you have both hands free, and don't have to take your eyes off the print to look at a timer. I wanted to try this, but ran into some complications.

All of the electronic metronomes that I could find ran off batteries. That was a problem because my footswitch is controlling power to the enlarger lamp. So I designed an isolator circuit that allows you to safely control the metronome indirectly from the power feed to the enlarger.

See the attached diagram. The LTV-817 is an LED-phototransistor device that provides 5000 volts of isolation between the input and output sides. When the enlarger lamp is on, current flows in the LED and the phototransistor turns on, operating the PRMA 1A05 relay. The relay contacts are in series with one of the wires to the loudspeaker in the metronome. This way, the metronome free-runs once it's set. The one I have forgets the tempo setting when power is removed, so it's easiest to control the sound by disconnecting the speaker.

To use the circuit, turn on the metronome and set it to 60 beats per minute. The circuit will cause it to sound only when the lamp is on. Turn off
the metronome when you're done printing so as not to run the battery down.

My enlarger has a Chromega colorhead that runs off 24 VAC, so R1 is sized for that. (Connect to the RED and WHITE wires in the cord to the colorhead.) If you want to adapt the circuit for a lamp that runs off of 120 VAC, change R1 to an 8.2K 5W part. Be aware that if you do that, you don't want to be able to accidentally contact the wiring on that side as it presents a real shock hazard. Make sure it's safely tucked away in a non-conductive enclosure. R1 will get a bit warm when current is flowing in it, so be aware of that as well.

If your metronome runs off 6V, you'll need to lower the value of the 470 ohm resistor so that the relay has ~10 mA of coil current to operate.

All of these parts are inexpensive and readily available from suppliers in the US like Digi-Key and Mouser.

[jbenedict]

Why not just let the metronome run? Once you get used to the sound, it becomes a part of the background until you concentrate on hearing it.

The batteries will last at least a year- probably longer. Should be easy to remember to turn it off. That ticking sound? That's the metronome.

[Jim Jones]

A cheap quartz clock that ticks once a second works for me. When not timing an exposure, I can drown it out with music.