LEDs are typically used to indicate the power-on condition of a circuit or a system. The circuit shown flashes the LED briefly (to get your attention) and stays on as long as the power is on (Fig. 1). The venerable 555 timer IC can be put to use in this type of application.
The circuit is a standard astable multivibrator with resistors RA, RB, and the timing capacitor C1. To operate as a free-running astable, the reset pin 4 should be held at VCC to disable reset. With the addition of R2, C2, and Q1, a delayed reset occurs that makes LED1 flash and then remain on. The circuit functions as follows:
At the instant VCC is turned on, C2 is fully discharged, Q1 is off, and reset pin 4 is at VCC. The circuit operates as an astable and C1 begins to charge through RA and RB. Once V6 reaches 2/3 VCC, C1 begins to discharge through RB. Once V6 reaches 1/3 VCC, the capacitor begins to charge again. The voltage across C1 oscillates between 2/3 and 1/3 VCC (Fig. 2). During this time LED1 continues to flash.
Meanwhile, C2 charges through R2 toward VCC. The diodes D1, D2, D3, and VBE (Q1) provide a turn-on voltage (VR) of 2 V. Once VC2 reaches VR, Q1 turns on and resets the timer. The output voltage V3 goes low and LED1 stays on until the power is turned off. If the LED is connected between pin 3 and ground, it will flash and stay off. The starting charging time (ts) to charge C1 from 0 to 2/3 VCC is:
ts = (RA + RB)C1 ln(3) = 1.1(RA + RB)C1 = 220 ms (1)
The discharge time (td) from 2/3 VCC to 1/3 VCC is:
td = RBC1 ln(2) = 0.69 RBC1 = 69 ms (2)
The charging time (tc) from 1/3 VCC to 2/3 VCC is given by:
tc = (RA + RB)C1 ln(2) = 0.69(RA + RB)C1 = 138 ms (3)
The total cycle time (T) during which the LED flashes is given by:
T = R2C2 ln \[VCC/(VCC − VR)\] = 995 ms (4)
The end time (te) is the difference between T and the various on-off times:
te = T − ts − 3tc − 4td = 995 −220 − 3(138) − 4(69) = 85 ms (5)
This attention-grabbing LED driver circuit finds numerous applications in frontpanel LEDs on instruments (such as spectrum analyzers, oscilloscopes, signal sources, etc.) which contain a cluster of push buttons to enable or disable a function, power-on indicators, warning lights, and laser pointers.