How many times have we heard "hey you could have just replaced that microcontroller with a 555"? Maybe 555 times! But we always find wiring up a timer chip to be a bit of a pain, you need quite a few components - especially if you want a buffered output.

That's why we designed the Adafruit 555 PWM Output STEMMA - a fully-assembled 555 timer board with adjustable PWM square-wave output.

We originally came up with this idea when someone requested we make a 'PC Fan Dummy' board, that can generate a 100Hz signal for faking a PC fan's tachometer signal to a motherboard. But then we thought that sometimes we want a square wave for driving a piezo, or as an audio input, or to modulate an IR signal. So we made it a little more general-purpose.

You can select between two 'ranges': approximately 1.4K~25 KHz or 1.4~250 Hz. The onboard trimmer pot will tune the actual output between those values. You can use a logic analyzer, oscilloscope, multimeter with frequency-counting, or even an audio input jack on a computer, to determine the precise value if you need more than a rough number. The output is a DC square wave, driven by a 74HC1G04 buffer so it can sink/source up to 12.5mA.

You can power the board from 3V to 12VDC, an onboard regulator will pin the input to the 555 and buffer to 5V max. That means that between 3V and 5V power, the signal peak will be at the same logic level as the power. Above 5V, the output will be pegged to 5V.

Each STEMMA board is a fully assembled and tested PCB but no cable. No soldering is required to use it, but you will need to pick up a 2mm pitch, 3-pin STEMMA JST PH cable. Alternatively, if you do want to solder, there's a 0.1" spaced header for power/ground/signal.

Power Pins

• VIN - this is the power pin. To power the board, provide 3V to 12VDC. There is an onboard regulator that will pin the input to the 555 and buffer to 5V max. That means that between 3V and 5V power, the signal peak will be at the same logic level as the power. Above 5V, the output will be pegged to 5V.
• GND - common ground for power and logic.

Signal Output

• SIG - this is the output signal from the 555 timer. It is a DC square wave, driven by a 74HC1G04 buffer so it can sink/source up to 12.5mA.

STEMMA JST PH

• STEMMA JST PH - 2mm pitch STEMMA JST port for use with 3-pin STEMMA JST PH cables. It has connections for:
  • GND - common ground for power and data. It is the black wire on the JST PH cable.
  • VIN - power input for the 555 timer. It is the red wire on the JST PH cable. 
  • SIG - signal output from the 555 timer. It is the white wire on the JST PH cable.

Range Selection Switch

On the top right of the board is a slide switch labeled 250 Hz and 25 KHz on the board silk. This switch selects between two 'ranges' for the 555 timer output: approximately 1.4~25 KHz or 1.4~250 Hz.

Frequency Adjustment Trimmer Pot

At the end of the board, opposite the JST PH port, is a trimmer pot. It is labeled Freq. Adjust on the board silk. This pot adjusts the actual frequency output from the 555 timer between the value range that you select with the selection switch.

Signal LED and Jumper

• Signal LED - to the left of the JST PH connector is the signal LED, labeled Sig. It is the red LED. It will light up when a signal is output from the 555 timer.
• LED jumper - in the center of the back of the board is a jumper for the signal LED. It is labeled Sig on the board silk. If you want to disable the signal LED, cut the trace on this jumper.

Power LED and Jumper

• Power LED - to the right of the JST PH connector is the power LED, labeled ON. It is the green LED.
• LED jumper - in the center of the back of the board is a jumper for the power LED. It is labeled On on the board silk. If you want to disable the power LED, cut the trace on this jumper.

You can use a logic analyzer, oscilloscope, multimeter with frequency-counting, or even an audio input jack on a computer, to determine the precise value from the output of the 555 timer. This breakout lets you adjust the frequency of the output from the 555 timer. The output is always a 50% duty cycle square wave driven by a 74HC1G04 buffer.

Wiring

You can power the board with 3V to 12VDC. You can connect the SIG pin to your chosen analyzer input. The following is the board wired up using a JST PH cable:

The following is the breakout wired up using a solderless breadboard:

Analysis

After the 555 is powered up, setup your chosen analysis tool. Select your frequency range with the slide switch and then fine-tune the frequency using the trimmer pot on the board. You'll see the frequency change on your tool as you adjust these settings on the board.

Files

• 555 Datasheet
• EagleCAD PCB Files on GitHub
• Fritzing object in the Adafruit Fritzing Library

Schematic and Fab Print

Here's how to use the 555 PWM Output STEMMA to trick your PC into thinking there's a fan running. This is useful when you've replaced the air cooling fans on your CPU or other parts of the system with liquid cooling radiators and pumps, such as a water cooled or oil cooled rig.

Some motherboard and BIOS combinations may not allow you to disable the fan alarms and may not allow you to boot past the POST test without a fan (or fan dummy) in place.

Since the fan headers on the motherboard contain a signal pin that watches for 5V square wave pulses, we can plug the 555 PWM Output STEMMA in and tell a big fat lie to the motherboard!

Just Right

There we are, 1025 RPM ought to keep everyone happy and now you can get on with your liquid cooling dreams! All thanks to a venerable 555 timer and a little bit of fibbing.