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Figure 4 - This LED pulsing circuit sends 800 milliamp short duration pulses
Figure 4 - This LED pulsing circuit sends 800 milliamp short duration pulses

The schematic shown in Figure 4 is a basic LED pulser that can send 800 milliamp pulses to a few LEDs, driving them with between 9 and 12 volts. When pulsing LEDs, the voltages used go well beyond the specified forward voltages shown in the datasheet, and pulse on times (duty cycle) must be kept very low. This circuit plays it safe, sending a very short repeating pulse of no more than 10 microseconds from the LM555 timer out to the LEDs, with as much current as the NPN transistor timer circuit can handle (about 800 milliamps). As you add more LEDs in parallel, the current required will increase until the transistor can no longer supply all of the LEDs. Depending on the specification of your LEDs, you may be able to drive up to four LEDs, or not even push a single LED to its capacity. An LED with a maximum pulsed mode current rating of approximately 1 amp (1000 milliamps) is about all the transistor can drive.

Since the initial experimentation will be done using visible LEDs, it is best to start with at least four LEDs in parallel and then pull one out at a time, checking the overall brightness as well as testing the transistor for heat with your finger. The 10 or 20 ohm resistor (R2) will limit the current enough to save your transistor from a meltdown, but the circuit should not be allowed to run hot continuity or the life expectancy of the transistor will be short. This initial circuit is just a test circuit, and will demonstrate the difference between continues low current as compared to pulsed mode current in the visible LEDs. Later, a larger transistor will be added in order to drive a much larger infrared LED array.



Figure 5 - The LED pulser test circuit is initially built on a solderless breadboard
Figure 5 - The LED pulser test circuit is initially built on a solderless breadboard

If you can't find a 2N2222 or 2N3904 transistor, then you can substitute practically any small signal NPN transistor that will handle current of around 600 milliamps. As for the 555 timer, any of the variants of this device will work, but be aware that altering the timing capacitor or resistor values will also change the frequency and duty cycle of the pulses. As it is, the circuit is set to deliver the quickest pulse that it can (around 10 microseconds), and the frequency of the pulses will be approximately 1.5 Kilohertz. You can certainly alter these values, but be aware that an error will usually mean a fried LED or transistor.

The LED pulser circuit is shown built on a solderless breadboard in Figure 5, using the values from the schematic shown in Figure 4. The LEDs used here are visible green LEDs with values very close to the infrared LEDs I plan to use later. My LEDs have a forward voltage of 1.2 volts, a continuous current rating of 200 milliamps, and a pulsed mode current rating of 1000 milliamps (1 amp). Because of the LEDs peak current rating being much higher than the transistor can supply, I know that as long as the pulses are kept as short as possible, no damage will occur to the LEDs. If the transistor failed and created a dead short, then the LEDs would also be fried, so it is important to keep checking the transistor for heat by touching it after a few seconds of operation.

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