# 1.5V 555 LED Driver Experiment

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.

Occasion for this experiment

Upon completing the recent Solar Rock Pathway Lighting evaluation, and reading the datasheet for the Zetex ZXSC380 LED driver, I realized that the 555 is quite similar and may possibly also do the job. I then measured the continuity between the open collector output ( pin 7) and Vcc (pin 8) and was pleasantly surprised to learn that there was no continuity to the positive rail – apparently, the open collector output is truly isolated and may be able to support the high (3V) inductive discharge that well exceeds Vcc. This builds upon a previous article: 555 Low Voltage Operation. In addition, it is always mind-stretching to find new, undocumented applications for the 555 oscillator – the application of a 555 with an open collector output is a favorite…

555 LED Driver Schematic

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.

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 (for comparison – I did not actually test this device)
Minimum startup and operating voltage: 0.9V typical, 1.0V max

Oscillographs

Photos

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).

Learned much from this experiment.

For the future

More on 555 open collector applications

##### Related Tutorials

• V.Sambath kumar

Hi Sir,

Thank you very much for your feedback.

Regards,
V.Sambath kumar

• V.Sambath kumar

Hi Jim Keith,

Sir,
I have read in some article about 555 ic that it can deliver a max
of 200ma current .so if we parallel pin3 and 7 what will be the max
current the ic can support assuming the in put source capable of
suppling 500ma current at a supply voltage 12v or 9v.

Regards V.Sambath kumar.

• Jim Keith

This cannot work because the totem pole output (pin 3) cannot support a voltage above the input voltage rail. Pin 7 can do this to a limited extent in that it seems to work OK at the 1.5V level.

Your best bet is to invert operation and drive an N Channel MOSFET with pin 3. Check out this circuit for inverted operation: http://www.electroschematics.com/7114/inverted-555-timer-circuit/

Of course, you may simply add a 2nd 555 timer that performs the function of an inexpensive inverter –think I will write a simple article on this relatively unknown feature…

• Mike 6250

Jim,
Nice this is my first learning project using a 555. Great tip. Pete nice catch and trying to help a new guy like me out of making smoke LOL
Mike

• Tiana

dadash sharmande kardi dari koaakmm mikonia.bebakhshid. bashe ina ro montaghel mikonam.vali mikham bedonam chera vase nasbe patch azam masir nemikhad? bad shoma file be name kitserver mishnasi? man nadaramesh.

• EZE DAVID UC

• Pete

Jim, this was a great article, and the details were great to read!

However, you have 555 pins 3 and 7 swapped on your schematic!

Pin 3 is the output, and pin 7 is the threshold input.

I hope this helps.

• Jim Keith

Thanks Pete for your positive comments. However, I am actually using pin 7 as an open collector output –I do this frequently and it works well –read the text carefully. In my 555 relay driver post, I actually parallel pins 3 and 7 to increase the current sinking ability.

• Jim Keith