Soldering Workshop - Circular LED Chaser

A circular LED chaser powered by astable 555 timer and 4017 decade counter.

The Spark Design Club held its first soldering workshop since the pandemic in January 2023, introducing around 40 engineering students to the underappreciated, surprisingly useful and occasionally white-hot art of soldering.

The candidate design for this project needed to fit within some tight budget constraints, require a variety of soldering/electronics techniques to accomplish, include a user design/customization element, and look great! We settled on a simple LED chaser project, utilizing the ever popular 555 timer in astable configuration to create a clock signal, which was fed into a 4017 decade counter to produce one hot outputs mod 10, which lit up 10 LEDs in a circle. A few supporting components were used as well, including the two resistors and capacitor for astable 555 configuration, a current limiting 0603 SMD package resistor between the LEDs and ground, and a 9V or CR2032 3V battery + wires to power it all.

This design satisfies our goals for the project:

- The 4017 and 555 ICs provide an example of DIP (Dual Inline Package) through hole components

- The capacitor and axial resistors provided examples of simple passives used in many projects

- The axial resistors offered configurability to the design. The clock frequency was inversely proportional to resistance, so selecting lower value resistors would increase the speed of the cycle. Unfortunately, trimpots were out of our price range.

- The 0603 resistor provided an example of an SMD component, though this was challenging for many to solder.

- The power wires demonstrated how to strip, tin and cut wire

- The LEDs offered more customizability, as we had many colours and sizes and people combined them into interesting patterns. It also provided an introduction to component polarity, and an introduction to desoldering for those who got it wrong.

Prices in Canadian Dollars

- PCB ($18 for 30 boards from JLCPCB - https://jlcpcb.com/PCH)

- 555 Timer IC, 8 DIP package ($.75)

- 4017 Decade Counter IC, 16 DIP package ($.75)

- Axial resistors. See steps for resistance calculation. ($.05)

- 2.2uF electrolytic capacitor ($.10)

- 0603 200ohm resistor ($.01)

- 10 5mm or 10mm LEDs, various colors ($1)

- 9V or 3V CR2032 coin cell battery

With the help of JLCPCB, we spent about $60 for parts to manufacture 30 boards, coming out to $2/board, not including batteries.

PCB sch file - https://drive.google.com/file/d/1BJZN-hRMVnYeiNJCk1ycZ5yD7EHNzBSs/view?usp=sharing

(For some reason, I cannot upload the .sch to instructables, but the accompanying .brd works fine)

Solder down the 0603 resistor first. It's easiest to do SMD components before THT, since the board lies flat on the table!

Insert the ICs into the PCB - make sure to check the polarity before soldering, as these will be very hard to undo. Then, solder in one leg at a time. No need to bend the legs, since they are rather brittle.

Calculate the resistance needed for the desired frequency. The equation, given equal resistances R = R1 = R2 and a 2.2uF electrolytic cap, is F = 1/(ln2 * 3R * 2.2uF). For example, if R = 3.6kOhm, then we get F ~ 60Hz. Since we have 10 LEDs, they will flash in a circle at 60/10 = 6Hz. That is, they will complete six loops a second.

Ref: https://en.wikipedia.org/wiki/555_timer_IC#Astable

Select the resistors R1 and R2 based on step 3, as well as a 2.2uF capacitor and solder them. Take note - electrolytic capacitors have polarity due to their chemistry. Reversing polarity on them can cause explosions. Insert the long leg into the + side.

Select and insert the LEDs (I recommend bending the legs in opposite directions to keep them in place). They need to have 5mm spacing between leads to fit. Ensure the polarity is correct (flattened circle edge is negative)!

Cut two ~10 cm wires for power, twist the ends and solder them into the holes in the top left corner of the board. Connect the ends to a battery. The top lead should go to +, I mistakenly didn't mark it on the board.

After soldering, the board worked perfectly for most people, and they got a souvenir of their new skill! A few issues were encountered with reverse polarity and poor joint quality, but these were solved relatively easily.

Thanks again to JLCPCB [ https://jlcpcb.com/PCH ] for sponsoring this project. They made this project feasible for us, to fund as we had to cover all costs with our club budget.

The boards delivered by JLCPCB were high quality and offered at an insanely cheap price, around $.60 CAD/board (which JLC generously covered, along with shipping. Thanks!). Spark has ordered PCBs from JLCPCB many times in the past, and they can be found in several of our projects. These boards have always been free of defects and worked as expected, with much lower costs compared to domestic (North American) manufacturers. Also, the blue soldermask is a beautiful azure shade, and fits our club theme well.