Interactive electronics resource
555 Monostable SMD Trainer Instructions
Build, understand and experiment with the 555 Monostable SMD Trainer. This resource brings the original kit material into the current MitchElectronics format with the schematic, circuit explanation, component and PCB information, construction guidance, troubleshooting and practical ideas where available.
Introduction
What is the 555 Monostable SMD Trainer?
The 555 Monostable SMD Trainer kit is designed to test your skills at SMD soldering and provides you with a small circuit that blinks for a period of time once pushing the button. The length of time the LED remains on is determined by the value of the electrolytic capacitor and the setting of the potentiometer such that the smaller the size of the capacitor or resistance value of the potentiometer, the period will be smaller. To learn more about the 555 timer and how it works, you can check out the instructions for the 555 Monostable kit from MitchElectronics which also includes the internal circuit diagram of the 555 itself.
What can you do with the 555 Monostable SMD Trainer?
Besides the obvious use of improving your skills with soldering SMD parts, there are numerous uses for the kit. One potential use for the monostable is to act as a bolt lock mechanism for a door such that entering the correct code causes a bolt to briefly retract. If the door isn’t opened within a set time delay, the monostable re-engages the lock to prevent it from being left open.
Another use for the 555 Monostable is to control the spout of an automated drinks dispense. A small DC pump connected to a motor driver and 555 monostable can be configured to deliver exact amounts of fluid, and these can include water, juice, and fizzy drinks.
Finally, the 555 monostable can also be used to control a power relay that enables a mains device to operate for a specific amount of time before automatically turning it off. For example, a games console can be connected to a monostable controller which prevents kids from playing for too long, and another example would be a sprinkler system that only remains on for a set period of time once activated.
Schematic
Try it yourself
Circuit simulation
What you need
Component List
| Component | Quantity | PCB Reference |
|---|---|---|
| NE555 | 1 | IC1 |
| 100Ω Resistor | 1 | R2 |
| 1KΩ Resistor | 2 | R1, R3 |
| 100KΩ Potentiometer | 1 | RV1 |
| 100nF Capacitor | 1 | C2 |
| 100uF Capacitor | 1 | C1 |
| Tactile Switch | 1 | SW1 |
| Red LED | 1 | D1 |
| PP3 Connector | 1 | +, - |
| 555 Monostable SMD PCB | 1 | — |
Inspect the board
Interactive BOM
Board reference
PCB & assembly
The PCB silkscreen and component references should be checked against the component list before soldering each part.
Before applying power
- Check every component against its PCB reference.
- Confirm the orientation of all polarised components and ICs.
- Inspect for solder bridges, unsoldered pads and clipped leads that could cause a short.
- Check that no loose wire or solder debris remains on the board.
Build with confidence
Construction tips
Recommended build approach
555 Monostable SMD Trainer uses surface-mount components, so work methodically and inspect each joint as you go.
- Check the component values and package markings before soldering.
- Start with the smallest passive components and work towards larger devices and connectors.
- For multi-pin devices, align the package carefully and tack one pad first; correct the alignment before soldering the remaining pins.
- Use only enough solder to form a clean joint and watch closely for bridges between adjacent pads.
- Observe the orientation marks on diodes, LEDs, ICs and any other polarised components.
- Inspect the board under good light or magnification before applying power.
SMD soldering guidance
If you are new to surface-mount assembly, use the SMD Soldering Guide alongside these instructions. Keeping the tip clean, using flux where appropriate and checking alignment frequently makes the build much easier.
When it does not work
Troubleshooting
Nothing happens when power is applied
- Confirm the supply is connected to the correct input and with the correct polarity.
- Check that ICs, diodes, LEDs, transistors and electrolytic capacitors are fitted in the correct orientation.
- Compare component values and positions against the component list and PCB reference.
- Inspect for missed joints, dry joints and accidental solder bridges.
- Closely inspect fine-pitch and adjacent SMD pads for solder bridges or unsoldered ends.
The circuit powers up but does not behave as expected
This kit is intended for smd 555 monostable timer trainer. If the output is stuck, unstable or outside the expected behaviour, use the schematic to trace the circuit a stage at a time rather than replacing several parts at once.
Check the components around the part of the circuit responsible for the output or timing first. A misplaced resistor, reversed semiconductor or poor connection can allow a circuit to power up while preventing it from operating correctly.
The circuit works intermittently
Intermittent behaviour is often caused by a marginal solder joint, a loose connector or a component lead that has not been fully soldered. Gently inspect the board with power removed and reflow any joint that looks dull, cracked or incomplete.
If the fault remains, compare the assembled board with the schematic and PCB reference one connection at a time.
Ready to test and experiment?
To help keep the board stable when soldering, you can download a free STL model of a basic jig that can be 3D printed with all common 3D printers. Watch out for the mounting hole pins as they may be vulnerable to snapping if using a low infill density, low wall thickness, or thick layer heights. Additionally, do not use hot air to solder the PCB when using the jig as you will melt the jig. Download 555 Monostable SMD STL Holder
Once the board is working, compare its behaviour with the schematic and the explanation above. Try changing only one input, control or permitted component value at a time so you can clearly see what effect that change has on the circuit.