#860 555 Upside Down

Testing an interesting upside down astable 555 configuration that achieves +duty cycle control all the way down to 0% without an additional inverter stage.

Build

Notes

The “555 upside down” is a variation of the typical 555 astable configuration that was presented in Elektor 8/2026, page 50.

It positions the charging capacitor on the high side of the 555 comparators, allowing duty cycles down to 0%. The typical minimum duty cycle in the standard astable configuration is about 50% - see LEAP#016 555 Timer - A-Stable Oscillator.

This is an interesting technique for achieving low +duty cycle control, without the need for an additional inverter stage after the timer.

Circuit Design

Designed with Fritzing: see UpsideDown.fzz.

I ended up varying the published schematic for a number of reasons:

I substituted a 1N5819 for the BAT85 as I don’t have any on-hand. The 1N5819 should have a forward voltage performance almost as low as the BAT85
Experimentally, I determined R1 should be around 4*R2 to allow the pot to be effective over most of its range. With the original R1=10kΩ and R2=5kΩ, the duty cycle control only covered about 20% of the pot’s range.
C1 and R1 can be adjusted depending on the application:
- lower capacitance (such as 100nF I ended up using) is good for frequencies above visibly detected blinking e,g, for LED dimming
- higher capacitance can put the output into the visibly detectable e.g. for LED blinking