Playing with fire

The Barton BMC78 Infrared Input is one of those modules that realistically you probably would not use much, but it’s fun enough to want to build it anyway. It’s just an infrared phototransistor, AC or DC coupled to a buffer, with a CV to gate function. Check out the demo.

I built the circuit on a breadboard and it worked, but doing that gave me some ideas for changes. Some pretty minor. I decided I liked an audio taper pot rather than linear for the sensitivity control (what Barton calls “bias”, but I think “sensitivity” is a better description), and for the gate control Barton evidently thinks of it as “gate rate” while I think of it as “gate threshold”, so I prefer it connected the other way around.

More significantly, I did try out the candle as random CV source idea

and a problem there, besides getting wax all over, is that setting up a candle a few inches in front of your synth is maybe not so prudent, especially if you want to turn any nearby knobs meanwhile. I figured adding a jack to connect an external phototransistor on the end of a cable, and a switch to select between the external and panel transistors, would be a good idea. Another problem is that the candle is a relatively bright IR source with relatively small fluctuations on top. If the sensitivity control is turned up enough you get a 10 V DC signal with variations of a few hundred mV. (If the sensitivity control is turned up further you get a 12 V DC signal with no variations at all, because the phototransistor is saturated.) You’re probably not interested in the DC level, just the variations — not the flame, just the flicker — so you’d want to use the AC coupling, but then your output is just that few hundred mV worth of fluctuations. I figured an amplifying stage rather than just a unity gain buffer was called for, to bump the output range up to a few volts. I added a gain pot to control the amount of amplification.

On the other hand, if you’re in DC mode, you probably do not want amplification, or at least not much… maybe a little bit if you’re dealing with a very faint IR source. So I made the AC/DC switch a double pole that also changes the amplifier gain range. In AC mode the range is 1 to 11. Not because I’m a rabid Spinal Tap fan, but because 11 is what you naturally get with a non inverting op amp configuration using a 100k pot and a 10k fixed resistor. In DC mode it’s 1 to 2.

I also felt the module could benefit from a couple more LEDs. One is an output CV indicator, a bicolor LED with an op amp driver so its brightness corresponds to the CV amplitude. Its color reflects the sign. (The CV is always positive in DC mode, but either sign in AC.) The other is driven by a comparator connected directly to the phototransistor, regardless of AC or DC mode, with a threshold of about 10 V, so it lights up when the transistor is close to saturation. This is to help in setting the sensitivity. Normally you’d probably want to adjust the sensitivity control to light up that LED and then back it off just a bit.

Even with these additions it’s still a pretty simple circuit. With all the panel components board mounted, it’ll all fit comfortably on a single PCB, plus a little board for the jacks and two of the LEDs, behind a 50 mm wide Kosmo format panel; there’ll also be a tiny PCB holding a phototransistor and a 3.5 mm TRS audio jack, for those remote candle setups.

One thought on “Playing with fire

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