1.5 V LED Driver Circuit

This 1.5V 555 LED driver is more of a fun learning experiment than a practical circuit. While the TLC555 can indeed drive a 3V white LED using the inductive discharge technique, it typifies the kind of problems present in low voltage circuits. Two circuits are presented: Basic 555 LED Driver and Bootstrapped 555 LED Driver.

Circuit function

When the output (pin 7) goes low, it charges L1 and when it turns off, the voltage developed by the inductor discharge easily causes the voltage across the LED to increase to its threshold voltage and subsequently flow through the LED.

555 LED Driver Schematic

Bootstrap mode

Note that operation of the toggle switch is necessary to shock the device into oscillation at marginally low voltages – otherwise, it cannot start. What happens is that when the switch closes, C2 charges through L1 and rings up double the incident input voltage – this is the effect of series resonance. Rectifier D2 and capacitor C2 then hold Vcc at a higher voltage level until oscillation commences and L1 starts repetitively firing the LED at 3V thus keeping C2 charged. This is a very interesting part of the experiment.

Not all 555s are created equal

To make this LED driver circuit function, the 555 must operate well below the specified minimum Vcc. Also, the maximum voltage specification of the open collector output (pin 7, discharge function) is not specified in relation to Vcc (pin 8). The first TLC555 worked so poorly under these conditions that I decided not to use its data. The particular LM555 was selected because I knew that it functioned OK at very low voltages. So a word for the wise – whoever attempts to replicate any of this stuff, will need a bag of 555s.

Results, Fig 1

Minimum voltage TLC555: 1.36V (some LED current as low as 1.0V – self-starting @ 1.0V)

Minimum voltage LM555: 2.30V (no LED current below this point – self-starting @ 2.3V)

Results, Fig 2, bootstrap mode

Minimum voltage TLC555: 1.13V (some LED current as low as 0.56V, self-starting @ 1.1V)

Minimum voltage LM555: 1.78V (some LED current as low as 0.80V, self-starting @ 2.5V)

Datasheet ZXSC380 

Minimum startup and operating voltage: 0.9V typical, 1.0V max

Conclusions

While the TLC555 can be made to work (sometimes and after a fashion), the ZXSC380 is a better choice having been designed specifically for this application. However, the ZXSC380 is available only in the tiny SO-23 package – not experimenter friendly – check out the SO-23 to protoboard adapter photo. Problems include poor low voltage operation and poor starting at low voltages. The selected TLC555 worked great at 1.5V, but poorly at 1.2V (Ni-Mh battery voltage).