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** Q_{1} and Q_{2} 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 R_{CH} and D_{1S} 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, T_{L}, of the generator is expressed as where V_{HYST} is the hysteresis voltage at the input of IC_{1} and V_{CC} is the supply voltage of IC_{1}.

For V_{CC}=4.5V, the typical value for V_{HYST} is **0.75V.** For the required value of T_{L}=0.5 sec, a value for R_{T} 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, T_{H}, is shortened by speeding up the time to charge capacitor C. This is done through the additional resistor, **R**** _{CH},** and the series-connected Scotty diode, D

_{1S}. The forward voltage drop at D

_{1S}is no more than 200 mV and can be neglected. The

**LED**is on for approximately

**(1/100)×T**msec.

_{L}≈5The LED driver comprises a PNP bipolar transistor, Q_{1} , and an NPNbi polar transistor, Q_{2} . Q_{1} and Q_{2} form as current source. At a high logic level at the cathode of Scotty diodeD_{2S} , a constant current flows through the **LED** with a value of roughly I_{O} ≈0.7V/R_{S} ,or about 10 mA in this circuit.

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