2016-08-20 Blue Blinky Gets A Different Timing Capacitor

It has been a long time since I designed Blue Blinky; I just wanted to update it.  I have used a 1 uF electrolytic capacitor for the timing capacitor, and it works just fine with the two 680k resistors to give about 1 per second flash rate.

I got a bag of hundreds of multilayer ceramic capacitors rated at 0.56 uF, 200 volts. The ‘K’ tolerance is 10%, which is better than many electrolytic capacitors, which are typically 25% or more. Using these .56 uF capacitors should give a more accurate flash rate.

I unsoldered the 1 uF electrolytic capacitor from one of my Blue Blinky circuit boards, and installed the .56 uF. I removed one of the 680k resistors and put in a 1.2 Megohm resistor. The total is .68 M plus 1.2 M = 1.88 M.

I connected the modified Blue Blinky up to an AA cell, and put it on a table. The flash rate was not stable, but I thought that was caused by the daylight detector getting too much light, so I darkened the room. It blinked for awhile but after a few minutes the flashes got longer. Normally the flash is quick, with nothing in between, but this changed to where the flashes were on 1/4 or more of the time. The flashes got longer until the LED was on constantly, so I just left it alone. Later the next day, the flashes started up irregularly, then the flashes got shorter until the flashes are now short and stable, like it should normally be.

I checked the AA battery’s voltage, it was 1.19 volts. This is basically a dead battery. The normal voltage is about 1.5 volts, and when the voltage drops to 1.3 volts, most of the battery’s capacity has been used. But this battery has gone well below that.

The problem I see with this modified Blue Blinky is that normal battery voltages above 1.2 volts cause it to become unstable and go on continuously. I’m going to have to put it on a variable power supply and find out what’s causing this odd behavior. I think the first thing I’ll try is to remove the light sensing diode to see if it or its transistor is causing the problem.

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