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Step 3Add the blinking circuit.
On the bottom half of the circuit, I swapped out the simple LED for a 555 timer circuit to make the LED's blink. Using the values I selected for C2, R5, and R6 (using the instructions in the specification sheet for the chip), I get a blink rate of about 4 per second with the LED's being on for 2/3 the time.
The LED's I have are red and have a voltage drop of 2.0 volts each. I decided to put 3 in series since I'm using a 9V battery (but possibly a partly dead battery, or an 8.4V rechargeable). R2 limits the current through the resistors: figuring 9 volts total, 6 volts will drop across the LED's, and about 0.7 volts drop across the collector-emitter junction of Q3, leaving 2.3 volts across R2. Using a 100 ohm resistor, the LED's are driven at around ( V / R ) = ( 2.3 volts / 100 ohms ) = 0.023 amps = 23 milliamps of current.
I got a chance to test the circuit and found that the LED's would blink for more than 10 minutes before slowly fading to nothing. I was shooting for something more like 60 seconds. I decided to reduce the capacity of C1 by a factor of 10. Using a 47 microfarad capacitor yielded a more rational 30 seconds before it got dim and it completely shut off by 90 seconds. I decided about 100 microfarads would be good -- 60 seconds before it got dim and 3 minutes before shutting off completely.
The LED's I have are red and have a voltage drop of 2.0 volts each. I decided to put 3 in series since I'm using a 9V battery (but possibly a partly dead battery, or an 8.4V rechargeable). R2 limits the current through the resistors: figuring 9 volts total, 6 volts will drop across the LED's, and about 0.7 volts drop across the collector-emitter junction of Q3, leaving 2.3 volts across R2. Using a 100 ohm resistor, the LED's are driven at around ( V / R ) = ( 2.3 volts / 100 ohms ) = 0.023 amps = 23 milliamps of current.
I got a chance to test the circuit and found that the LED's would blink for more than 10 minutes before slowly fading to nothing. I was shooting for something more like 60 seconds. I decided to reduce the capacity of C1 by a factor of 10. Using a 47 microfarad capacitor yielded a more rational 30 seconds before it got dim and it completely shut off by 90 seconds. I decided about 100 microfarads would be good -- 60 seconds before it got dim and 3 minutes before shutting off completely.
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