I had heard some stories in the past concerning how poorly the 555 ran below Vcc = 5V, so I embarked on an endeavor to see if I could improve operation. The spec sheet specifies minimum Vcc at 4.5V and I tended to believe it. I set up a simple test circuit and ran each of 5 different bipolar 555 devices while varying Vcc. What I learned surprised me. Not only did it run substantially below 5V, but it ran well. Then I tried a CMOS 555 and that worked below 1V.
555 Test Circuit Schematic
555 Low Voltage Data
My attempt to improve upon perfection
Then I tried fiddling with the voltage at pin5 in an attempt to make it go even lower, but had very marginal improvement—not recommended!
Oscillograph of Fairchild 555 running at Vcc = 2.5V
Trace 1 clearly indicates the proper switching points of 0.833V and 1.667V, and shows the two RC slopes: charging slowly via two 10K resistors in series, and discharging more quickly via one 10K resistor.
Trace 2 shows the pin 3 output voltage as it sinks LED current (approx 20mA). Vo(low) indicates a saturation voltage of 0.7V. Vo(high) indicates an unloaded maximum output of 2.0V. The 47Ω resistor has minimal effect upon load current as the LED series resistance and output resistance of the 555 accomplish most of the limiting.
I did not attempt to connect the LED in the source voltage drive mode, but results should be similar.
Operating beyond the specifications — the twilight zone…
If it functions well below the specified voltage, why not specify it lower? Many issues are involved here. First, note that my sampling is very small—there are likely some that require a higher voltage to function. The manufacturer wants to produce a product that always works within specifications over the entire operating temperature range and include manufacturing tolerances. If operating in an out-of-spec condition, the manufacturer will take no responsibility for problems. Any decent engineer also has to know that there are liability issues at stake should such a product malfunction. However, for experimentation, the sky is the limit—as it is with experimental aircraft where the pilot accepts all liability…
The 555 in history
I first starting hearing about the 555 in the early 1970’s. The first technical presentation I attended was a PowerPoint titled “Triple Nickel Timer.” I think it was put out by Motorola and I was not all that impressed, not knowing that it would soon become an industry standard.
One vintage NE555N that I tested was manufactured by Signetics and bears the date code: 8116 (1981, week 16). Today, the 555 is perhaps the least expensive IC available at $0.174 each (quantity 1) or $0.073 each per 5000 pieces (TI, SOIC-8).
The 555 functions well below 4.5V and all devices tested functioned at 2.55V.
Practical Application of 555
At this low voltage, we are generally considering 3V logic or battery operation. While some may work with a 1.5V battery, there is insufficient voltage to directly drive an LED. So my suggestion is to run it no lower than 3V (two cells in series)—and remember that this is an out-of-spec-condition!
For the future
- 555 Inversion
- 555 VCO
- 555 MOSFET Gate Driver
- 555 Trivia
- 555 Charge Pump Voltage Doubler
- 555 Strobe Generator
- 555 Open collector output