Low-duty-cycle LED flasher keeps power draw at 4 mW

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 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 Schottky 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 IC₁ and V_CC is the supply voltage of IC₁.

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 Schottky 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_L≈5 msec.

The LED driver comprises a PNP bipolar transistor, Q₁, and an NPN bipolar transistor, Q₂. Q₁ and Q₂ form a switchable current source. At a high logic level at the cathode of Schottky diode D_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.

Series-connected silicon diodes D₁ and D₂ provide strong nonlinear negative feedback. If for any reason the voltage drop across the sensing resistor, R_S, rises, the D₁-to-D₂ connection will force almost the same increase in voltage at the emitter of Q₂. This increase reduces the collector current of Q₂ and, therefore, the base current of Q₁ and closes the loop; the net result is a reduction of the collector current of Q₁ to maintain a constant value.

Note that when the output of IC₁ goes low, the current through D_2S and resistor R_B is negligible. This is due to the fact that, with the output of IC₁ low, the base of Q₂ is held low, turning it and the current source off. With the current source off, the LED is off as well, and only microamps of leakage current flow through D_2S and R_B. If you use all surface-mount devices, you can build the circuit on a board no larger than 16×16 mm.

This work was supported by the Slovak Research and Development Agency under contract no. APVV-0062-11.