PAGE 080 OF 150 · TIMERS, FLASHERS & DIMMING
Two-Transistor Flasher
Two transistors can form a simple oscillator.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The circuit
Two NPN transistors are cross-coupled through capacitors, so when one switches on the other switches off, and they swap repeatedly. Each collector has an LED with a 330 ohm resistor, and each base has a resistor, such as 47 kilohms, to the supply.
Timing
With equal parts the full period is about 1.4 × R × C. For 47 kilohms and 10 microfarads, that is roughly 0.66 seconds, or about 1.5 flashes per second.
Notes
- Run from 5 to 9 V.
- The LEDs flash alternately.
- Larger capacitors or resistors slow it down.
REMEMBER
Switching an LED on and off quickly can look like steady dimmer light.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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