Battery-operated equipment often will benefit from a power-on indicator.
The indicator, however, can waste significant power. In situations where a low-duty-cycle blinking indicator provides an adequate indication of the power being turned on, the simple circuit described here should prove useful.
Figure 1 Q1 and Q2 function as a current source and push a constant current through the LED regardless of its forward voltage drop (within the compliant voltage limitations). The Schmitt inverter forms a classic square-wave generator, modified with RCH and D1S to produce an asymmetrical output.
A tiny, single-gate Schmitt-trigger logic inverter, the SN74AHC1G14, together with two resistors, a Scotty diode, and a capacitor form the timing generator of the blinker, shown in Figure 1. The output waveform has a period of about 0.5 sec and a very low duty-cycle value, of around 1%. The interval of low-output duration, TL, of the generator is expressed as where VHYST is the hysteresis voltage at the input of IC1 and VCC is the supply voltage of IC1.
For VCC=4.5V, the typical value for VHYST is 0.75V. For the required value of TL=0.5 sec, a value for RT of 200k was selected. The value of the timing capacitor, C, can be calculated from the equation, with a small amount of algebraic rearranging, as 7.45 μF. The nearest standard value is 6.8 μF; a tantalum solid-electrolytic capacitor is used for this value. To achieve the low duty cycle of the generator, the high-output duration, TH, is shortened by speeding up the time to charge capacitor C. This is done through the additional resistor, RCH, and the series-connected Scotty diode, D1S. The forward voltage drop at D1S is no more than 200 mV and can be neglected. The LED is on for approximately (1/100)×TL≈5 msec.
The LED driver comprises a PNP bipolar transistor, Q1 , and an NPNbi polar transistor, Q2 . Q1 and Q2 form as current source. At a high logic level at the cathode of Scotty diodeD2S , a constant current flows through the LED with a value of roughly IO ≈0.7V/RS ,or about 10 mA in this circuit.
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