Button Pusher for Time-Lapse Photography

Button Pusher for Time-Lapse Photography


This article is being cross-posted in:

sci.electronics.design sci.electronics.basics sci.electronics.misc alt.binaries.schematics.electronic

The related binary files are posted only in the .binaries group.

In case anyone else might be interested for future reference, this is as-built documentation of a project previously discussed in these threads:

sci.electronics.design: Remote camcorder controller 02 Jul 2007 Switching +/-12V from 6/0V 12 Jul 2007

My camcorder will take still pics as well as video, and it has a remote control. I wanted to send the remote's "Photo" signal to the camcorder at a regular interval so I could do what amounts to time-lapse photography. Earlier I considered more elegant ways to do this, but ended up just rigging up a button pusher for the remote.

The project was made possible by an item from my junque box - a rare Magnecraft open-frame relay, #W88KDX-2, which I used as a solenoid. The floating contacts travel just the right distance, and with enough force, to depress the button on the remote.

Basically, the remote and the relay are mounted on a strip of wood which is taped to the tripod handle (the camera is mounted backward so its I/R detector can see the remote's I/R LED). The remote is on top, and the relay on the bottom, and the two are connected via a hole drilled through the wood strip. Materials used include popsicle sticks, a short piece of wire-wrap wire (it must be blue), masking tape, a right-angle mounting bracket, and a rubber band. A Dremel tool with a cutting disk was used to cut off the fixed relay contacts and to cut the slits for mounting the connecting wire.

The drive electronics are on a breadboard, powered by 4 "C" cells, all of which just hang from the tripod. The circuit uses one TLC555 timer to provide the interval between button presses, and an LM555 timer to set the duration of each button press. A dipswitch allows selection of intervals ranging from about four seconds to over one minute.

Since the relay requires at least 12V, and a fair amount of current, I used a MAX232 chip to convert the incoming 6V from the battery pack to a much higher voltage the relay can use, and a large capacitor to store that charge between presses. The two-transistor coil driver was suggested by James in the .design group, but the resistor values are all my fault. Thanks to James and all the others there who helped me figure this out.

I don't have a schematic drawing program, so I just drew it by hand and took a picture. I hope it's readable. If anyone has questions or comments, fire away.

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If your relay coil resistance is really 100ohm, I don't see how that MAX232 can supply the needed 120mA. From the datasheet, the MAX232 can only supply about 10mA (short circuit), but this is with 1uF caps (not the 10uF that you use).

What voltage are you really getting across the coil when it's on?


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You are correct that the MAX232 can't supply anywhere near enough current on a continuous basis. But over time it can charge up a capacitor with enough juice so that when the time comes to push the button, the accumulated charge on the cap can be dumped all at once through the coil, and that's enough to make it work for a short time.

I use the Tx outputs of the MAX232, one high and one low, to charge the 220 uF cap, which process takes about three full seconds to complete after the previous discharge. You can watch the voltage across the cap ramp up with an analog voltmeter, just like the books say it should. At that point, it's charged to about 22V. And during that three seconds, the current drawn by the MAX232 is about 30ma maximum, which is pretty much in line with the data sheet.

I have no way to measure how much current the cap discharge actually produces, but it's enough to pull in the relay and push the button for a fraction of a second, which is all I need. So basically, it's really all about doing a slow, low-current charge of the big cap, followed by a sudden, hi-current discharge.

By the way, I don't think the 10uF caps make any difference. I used them because I had them and didn't have four 1uF's.

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