Introduction: Designing a 555 Timer on Discrete Elements

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I designed the circuit board for the NE555 timer and it works! Try it too!

Step 1: Making a 555 Timer on BJT

Hello! My name is Vladimir, I am a programmer and electronics developer. Analog circuitry has always been my weak point, and therefore the idea came to me to develop a tutorial on the internals of the NE555 timer.
You may have seen similar boards from Mad Evil Scientist, and others on this site. I think my version of the board will also be useful for enthusiasts like me.

Step 2: Adapting the Circuit for Assembly on Discrete Elements

As we recall, the 555 timer is relatively uncomplicated, and although each manufacturer contributes something different, the functional diagram will look the same.

As in many other circuits, diodes were replaced by transistors, which contributed to the unification of the circuit elements. Also, the resistances of the resistive divider were replaced with 4.7k resistors, as well as others were replaced with more common nominal values from the E12, E24 series.

For the output circuits Output and Discharge, the S8050 transistors were chosen as they can provide higher currents. The rest of the schema from the documentation remained unchanged.

Step 3: PCB Tracing

I really liked how the functional blocks of the device were divided on the Mad Evil Scientist board, and I repeated this trick. Unfortunately, it was not without errors, and only after sending the board to print, I noticed that the conclusions were mirrored relative to a real integrated circuit.

Step 4: PCB Assembly

PCB assembly is not difficult, even for inexperienced users .. but it still takes a long time.

The first step is to establish resistances. It is advisable to pre-form conclusions according to the size of the installation dimensions. Installation of transistors is allowed only after carefully checking them with a multimeter or a specialized tester. Because the components ordered from China are of course very cheap, but not reliable.

After assembly, it is recommended to rinse the board in an ultrasonic bath with warm isopropyl alcohol. Ventilate the area thoroughly and use respiratory protection.

Step 5: Testing

There are three test points on the board to check the functionality of the assembled timer. In the diagram, you can find them as TP1, TP2 and TP3.

At TP1, the voltage should be equal to 0.6 V when the voltage taken from the variable resistor is more than 2/3 of the supply voltage, and when less, it should jump abruptly to 0 V.

At point TP2, the voltage should be 0.6 V when the voltage taken from the variable resistor is less than 1/3 of the supply voltage, and when it is higher, it should jump abruptly to 0.2 V.

At point TP3, hysteresis should be observed: we smoothly raise the voltage on the AC voltage engine to 2/3 of the supply voltage, the voltage at the control point jumps to 1.44 V, smoothly decreases to 1/3 - it drops abruptly to 0 V.

Step 6: Checking the Timer in Operation

Once assembly is complete, you can plug in the board instead of the actual 555 timer. I used a simple tester circuit with blinking LEDs that I use to test all purchased timer ICs.

Project files
All detailed information can be found on the project page in easyEda https://oshwlab.com/kisly.va/big-555

What's next?
This is primarily a tutorial that we plan to develop and supplement: reduce the size, add test items with a "blinking LED".