A 555 timer-based metronome for bike commuters.
When the weather became cooler, I started biking to work. I ride through a beautiful trail passing through the city, it's nice to see people walking on it and enjoy the breeze while riding a bike.
But sometimes the fun has to pause because some people block the entire trial and not aware of me coming through. (sidewalk spreading)
So I thought to myself, why not build one myself?
I have heard so many good things about the 555 timers and always wanted to build something with them!
I started out with a demo video of 555 timer based metronome. After a quick search, I found this YouTube post, which has a rough schematic. I tried to build the same exact circuit, but unfortunately, I could not find the exact same parts. Taking a shortcut is always desirable but not always work! So I went back to the datasheet and study how exactly 555 timers work.
555 timer schematic from datasheet
Looking at the internal schematic of a 555 timer, it has 2 op-amps, 1 flip-flop, 1 transistor, 1 NOT gate, and a couple of resistors. How 555 could be a timer with these components? There are plenty of YouTube videos that explain this way better than me, but here is my attempt.
The short version of the story is the 555 timer charging up the external capacitor and using 2 op-amps to determine whether if it is fully charged. Once the capacitor is fully charged, the 555 timers will discharge the capacitor to empty and restart the charging process from the beginning. The above steps complete a cycle, and the period of the cycle can be found with the capacitance of the capacitor and the current limit resistor used to charge the capacitor.
Metronome with 1k resistor.
One thing I didn't understand from the metronome schematic was the 1k resistor between the VCC and discharge pin. So I proceed with my altered design which does not have this resistor. After powering on the circuit, I quickly realize the current is close to 80mA, it is 10 times higher than the recommended value in the datasheet. I also notice the 555 timer is a bit warm with that current flow. Turns out that it is unwise to draw a large current with a tiny built-in transistor. Added a 1k resistor, the current will be limited to a few milliamps.
After I had the entire circuit working on my breadboard, I quickly drew the schematic with KiCad. Once I figured out different footprints and packages, the routing did not take long. Plotted the Gerber files and sent them to the PCB manufacturer, and I'm excited to report that they took my files and successfully showed me the correct layout!
2 weeks later, I received the PCB in my mail.
My PCB layout on manufacturer's website
Received PCB
Parts all soldered
After soldering all the components, I was very excited that it worked as expected! I was able to tune the frequency with the potentiometer, and the buzzer is reasonably loud!
However, there are two things I didn't like about my first version.