555 timer drives multiple LEDs from one NiMH cell
|Click here to view a PDF of all the Design Ideas from this issue.|
Using a CMOS 555 timer and a single NPN transistor, you can drive as many as seven LEDs using a minimal amount of voltage and power from a single NiMH (nickel-metal-hydride) AA cell. The circuit works by creating much higher-voltage pulses than the voltage for powering the circuit by pulsing a high-Q power inductor. The circuit creates voltage pulses of 23V using a 1.25V NiMH cell with seven connected LEDs.
Figure 1 Using a CMOS 555 timer configured as a switching power supply, you can drive seven high-brightness LEDs from a single 1.25V cell.
The circuit uses a CMOS timer because it functions on low voltages—in this case, as low as 1V. A single white LED rated at 9300 mcd maintains its brilliance down to this low voltage. The circuit works for 192 hours using a 2000-mAHr-rated NiMH cell. The output of the timer is a 4.5-µsec pulse repeating at a 222-kHz rate. Although you can use the circuit to power any LED, it works best using high-brightness, high-power LEDs rated at 3000 mcd or higher. Obviously, the higher the millicandela rating, the brighter the LED will appear.
You can connect the LEDs in parallel if their forward voltages match; otherwise, the LED with the lowest forward voltage will dim out the other LEDs. Using the parallel connection, all LEDs will glow with equal brightness if their forward voltages match. Adding LEDs does not increase the current drawn from the battery but reduces the brilliance of all of the connected LEDs.
The advantage of connecting the LEDs in series—which is possible because of the high pulse voltage they produce—is equal brilliance of all LEDs, regardless of their individual forward-voltage drops and millicandela ratings. Each additional LED decreases additional voltage and lowers the resulting current into the series string of LEDs, lowering their brilliance. Using seven LEDs with a single 1.25V cell draws a current of only 8 mA. By adding a 1.25V cell to the power input, the LEDs become so brilliant that it is difficult to look at them. With a 2.5V supply, the peak voltage pulses increase to 70V with no connected LEDs. With the LEDs connected, the output voltage peaks at 25V. Current draw at 2.5V is 20 mA.