Blink Test
So there isn’t much I can do on the VU meter project until I get a hold of an oscilloscope. The next logical step would be to design the envelope follower that the comparator will receive input from, but doing this “blind” would be difficult. In the mean time I thought I’d check that the relay I’m using can actually switch as fast as I need it to (without burning up) and get a feeling for what rapidly flashed incandescent bulbs look like.
I wired up a simple astable multivibrator 555 timer circuit (which outputs a series of 5 volt pulses) whose frequency I could control with a potentiometer. I got a 6ft lamp extension cord, cleaved the two wires in two and cut one to put the relay in line.
The primary reason to do the test was to see how much latency the 100 watt incandescent bulbs display at various rates. The bulbs take a little while to get to full brightness after current starts to flow and even longer to become totally dark after it stops. A little bit of this is okay, and the trailing edge will actually produce a neat ghosting effect as the meter falls off. Too much latency though, especially in reaching brightness, will mean a dim top edge to the meter and a disappointing disconnect between sound and visual.
By playing around with rapid pulsing and watching it closely I don’t think it will be a problem. It’s difficult to tell without seeing several of these in a row though, so I’m going to try to measure the brightness curve from the following video and see if I can write a quick simulation (using processing?) of what the completed meter would look like with these bulbs. The camera’s auto-exposure function sensor latency will skew the data a bit, but it should be enough for a rough idea. Alternatively, when I get an oscilloscope (what can’t they do!), I could use a photoresistor to measure the brightness curve more precisely.
I’ve started a photoset for this project here.