The Smart, Stealth LED Blinkenlight

Step 2 Design a night-time vibration-sensitive circuit.

I figured I'd do an experiment to see if this circuit was any better. The idea is that if C1 is discharged and vibration switch S1 is off (no vibration) then virtually no current will flow through Q3. I measured the battery current (actually, using a 5V power supply, but it's rather irrelevant) with S1 open and C1 discharged. With a high-precision meter, I measured no current. Just for kicks I put in a 100 ohm resistor and measured 0 volts across it so I stepped way up to a 100,000 ohm resistor and managed to measure 0.0004 volts -- that's 4 nano-amps of current, so (assuming a battery that never dies on its own) a 9V battery with 540 milli-amp-hours of capacity would last around 15,000 years.

So it's basically no current when not operating.

Since EAGLE didn't have a cadmium-sulfide (CdS) light sensor already, I just used a photodiode for D1 instead -- it's really a CdS cell, though. The CdS cell has a high resistance in the dark (over 100 kilohms) and a low resistance when it's light (under 200 ohms).

Anyway, when S1 is pulsed on (at this time I just have a couple bare wires I tap together) the current draw in daytime is around 230 microamps. So riding around in the daytime, one can expect no less than (540 mAH / 0.23 mA) = 2350 hours = 98 days of continuous riding. Since S1 pulses on and off, it is more than that, and probably much more.

The interesting stuff happens at night -- when riding, perverts. Transistor Q1 will turn on, charging C1. Since the DC current gain (called "hfe" on specification sheets) of Q1 is around 150x, the 230 microamps through the base-emitter junction is amplified to about 35 milliamps through the collector-emitter junction and also through capacitor C1. C1 will charge to the full 9 volts in a fraction of a second.

So now, regardless of the vibration switch or night sensor, C1 begins to discharge through R2 through the base-emitter junction of transistor Q2. When C1 is fully charged to 9 volts, R2 limits the current to (V / R) = (9 volts / 10000 ohms) = 0.9 milliamps. However, since Q2 also acts as a DC amplifier with a gain around 150x, the current through it could be as high as 135 milliamps -- much more than is necessary to run the LED's alone, and when run through Q3 as well, the maximum current (again with hfe=150) skyrockets to saturation -- 20 amps is far more than the transistor could handle.

However, what if C1 is almost completely discharged? If it only has 0.1 volts in it, then the numbers work out to 10 microamps, amplified by Q2 to 1.5 milliamps, amplified by Q3 to 225 milliamps. Again, this is maximum current, but since the LED's will only draw 20 milliamps, that's the limiting factor in the current.

I tested the circuit as is and it functioned like I expected: the LED's would light up and stay on if CdS cell D1 is in the dark and S1 is pulsed. The LED's stay on for a while.