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Mini Projects #024 – by Tim Blythman SILICON CHIP Discrete 555 Timer The 555 is one of the best known ICs; it was designed over 50 years ago but is still in production and use. It has many uses beyond its original intent as a timer. Our circuit closely approximates the operation of the main features of the 555 timer, allowing classic 555 designs to be investigated. W e have seen circuits and even kits that attempt to be faithful to the internal workings of the 555; our intention with this circuit is to see how easy it is to implement the workings of an integrated circuit (IC) using just a few components on a breadboard. It is not a direct replacement for a 555, but it will allow many 555 circuits to be built and investigated. We have favoured simplicity over exactness. Our circuit does not have all the features of even the cheapest 555 chip. We’ve simulated and tested it at 5V, and we know that it works from about 4V to 6V. It should work at higher voltages, too, but we’re specifying some 10V capacitors, so you would need to change that for operation above 9V. It lacks a RESET input and pushpull OUTPUT, but these are not needed in the most common applications. It wouldn’t be hard to add them, but we felt they would detract from the simplicity. You can see our circuit in operation by watching the video at: siliconchip. au/Videos/Discrete+555 comprises a handful of components. Three identical resistors connected in series produce voltages at 1/3 and 2/3 of the supply. There are two comparators and a latch; these are the core components used for timing. In the typical astable configuration (Fig.2), the TRIGGER and THRESHOLD pins are connected to a capacitor, C. The capacitor charges via the two resistors until it reaches 2/3 of the supply voltage, triggering COMPARATOR 1. This activates the latch and thus the DISCHARGE transistor. The capacitor then discharges until its voltage (and thus TRIGGER and THRESHOLD) drops below 1/3 the supply voltage and COMPARATOR 2 is triggered. The latch changes state and the DISCHARGE transistor switches off, allowing the voltage to rise and the cycle to continue indefinitely. The 555 timer The block diagram of a 555 timer (Fig.1) is a good place to start. Even a simple IC like this has its own building blocks. Each of these blocks Fig.1: the 555 IC comprises these internal building blocks. Our version lacks the reset function and output driver, although it includes an indicator LED to show the output state. 20 Australia's electronics magazine Silicon Chip siliconchip.com.au Fig.3: it’s remarkable that the building blocks shown in Fig.1 can be reduced to two or three transistors and a handful of resistors. The real 555 has many more transistors, making it a lot more tolerant of supply voltage variations and other operating conditions. Fig.3 is our circuit, with the blocks marked to align with Fig.1. The three 10kW resistors in series create the 1/3 and 2/3 supply voltage references. The components at right are the extra ‘external’ components needed to set up the circuit as an astable multivibrator. Each of the comparators consists of two transistors and two resistors, with one comparator having an extra transistor to invert its output. In each comparator, the two transistors form a differential pair. All the current through the pair must flow through the top resistor, which Fig.2: just three external components are needed to turn the 555 IC or our circuit into an oscillator. siliconchip.com.au connects to the emitters and is split into separate collector circuits. The current through each collector will thus depend on whether each transistor is conducting. With their emitters at the same voltage and since the emitter-base junctions are effectively silicon diodes, whichever base is at a lower voltage will conduct substantially more of the current. That will switch on that transistor, allowing current to flow through the corresponding collector. Q2’s base is set to 3.3V by the divider. If Q1’s base voltage is lower than that, Q1’s collector will carry all the remaining current coming through the emitter resistor. No current flows through Q2’s collector, and it sits near 0V. If Q1’s base rises above 3.3V, then current flows down Q2’s branch instead, causing the voltage on Q2’s collector to rise due to current through the 10kW resistor. The other differential pair works similarly, although its output is instead fed into an inverter (Q7 and its collector resistor) so that the TRIGGER output goes high when Q3’s base falls below the 1/3 level. Instead of resistors, a real 555 IC uses current sources and current mirrors, allowing the circuit to work better over a wider range of voltages, but resistors are simpler. The latch Transistors Q5 and Q6 plus four resistors form a bistable latch. This Australia's electronics magazine is effectively a form of memory that retains its state unless it receives an external signal to change. If one transistor is on, it pulls the base of the other transistor low, forcing it off. This is positive feedback, reinforcing the current state of the circuit. To change the state of the latch, an external signal supplies base current to switch one of the transistors on, forcing its counterpart to turn off. Here, we use diodes to inject current from each of the differential pairs into either side of the circuit. The last thing needed to use our timer circuit in the classic 555 astable configuration is a DISCHARGE output. This is simply an NPN transistor in the same open collector configuration seen in Fig.1. We’ve also added transistor Q9 to drive LED1 to show the state of the circuit. It also helps to even out the load on Q5 and Q6 so that they behave symmetrically. Astable oscillator All that is needed to create an astable multivibrator (or oscillator) is to add the parts on the right-hand side of Fig.3; these are the same minimal parts needed to turn a 555 IC into an oscillator. They do a job very much the same as in an IC-based circuit. The capacitor starts in a discharged state, meaning that TRIGGER and THRESHOLD are both low. Importantly, the TRIGGER voltage is less than 1/3 supply, so the current flows April 2025  21 through D1, meaning that Q5 is on and Q6 is off. DISCHARGE (Q8) is off and the capacitor can charge through the resistors. Q9 and the LED are on. At 1/3 supply, the TRIGGER comparator stops supplying current to D1, and the latch keeps its current state. At 2/3 supply, the THRESHOLD voltage is passed and current now passes through D2, switching on Q6 and switching off Q5. DISCHARGE switches on too, and the capacitor discharges until 1/3 supply is reached. The cycle then repeats. Parts List – Discrete 555 Timer (JMP024) 1 breadboard or prototyping board [Jaycar PB8820] 4 BC557 100mA PNP transistors (Q1-Q4) [Jaycar ZT2164] 5 BC547 100mA NPN transistors (Q5-Q9) [Jaycar ZT2152] 2 1N4148 75V 200mA small signal diodes (D1, D2) [Jaycar ZR1100] 1 yellow 3mm LED (LED1) [Jaycar ZD0110] 11 10kW ¼W axial leaded resistors [Jaycar RR0596] 1 4.7kW ¼W axial leaded resistor [Jaycar RR0588] 1 2kW ¼W axial leaded resistor [Jaycar RR0579] 4 1kW ¼W axial leaded resistors [Jaycar RR0572] 2 220μF 10V electrolytic capacitors [Jaycar RE6157] 1 5V DC power supply Hookup wire or jumper wires Construction We laid our circuit out on a breadboard, since we expect readers will want to change the circuit to test out its operation. It could be transferred to a prototyping PCB like Jaycar’s Cat HP9570 instead. Our Parts List includes the wiring and the parts needed to use the circuit as an oscillator; Fig.4 shows the layout we used. Q1-Q4 are the PNP transistors; we used BC557s, but any of the BC55x series parts should work. Similarly, Q5-Q9 are BC547 NPN transistors that can be substituted with any BC54x equivalent. The red arrows show the external ‘pins’, with power and ground being supplied through the side power rails. All power links are shown in red, with ground in black. Other internal connections are blue. Note the power links at the top of the breadboard. The green wires and three components at upper left are the added components needed to turn the circuit into an oscillator. The values shown here should cause the LED to flash at a rate of about 1Hz. While building your version, you can also refer to our photos. If you don’t see anything happen when you apply power, check your wiring. You can probe the circuit with a multimeter to see what might be wrong. Testing We started by building our circuit in the LTspice circuit simulator. It is free to use and can be downloaded from siliconchip.au/link/ac2p We published a series of articles about LTspice in 2017 and 2018 (siliconchip.au/Series/317). It’s a great way to test out circuit configurations and values before going to the trouble of plugging components into a breadboard. You can try our simulation file to see how the circuit operates (download from siliconchip. au/Shop/6/1821). Scope 1 shows the output of the simulator. You can see the two comparators briefly activating in turn and toggling the state of the latch. The waveform is oscillating at 1.24Hz. The calculated frequency for a 555 timer in this configuration with these components is 1.32Hz. We suspect the reason our version is a bit slower than expected is that it slightly overshoots the 2/3 supply voltage threshold. If you are going to experiment, we suggest sticking with external resistors similar in value (around 1kW) to the ones that we have used. Conclusion The comparator and latch are very common building blocks in all types of circuits. Here, you can see how they can be combined to create a simple but flexible circuit that can SC do many jobs. Scope 1: our LTspice simulation of the astable multivibrator. The grey and purple traces are the 1/3 and 2/3 supply reference voltages, while TRIGGER and THRESHOLD follow the green trace (since they are connected together). The red and cyan traces are the outputs of the comparators that trigger the latch to change state. 22 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.4: this is a simplified version of the classic 555 timer IC that you can build from a couple of dozen components. Follow this diagram closely, since many of the components are close together. Observe the type and orientation of the transistors. The PNP transistors on the right have their emitters joined and thus they share a row. The ‘external’ timing parts are those at upper left plus the green wires. ourPCB LOCAL SERVICE <at> OVERSEAS PRICES AUSTRALIA PCB Manufacturing Full Turnkey Assembly Wiring Harnesses Solder Paste Stencils small or large volume orders premium-grade wiring low cost PCB assembly laser-cut and electropolished Instant Online Buying of Prototype PCBs www.ourpcb.com.au siliconchip.com.au Australia's electronics magazine 0417 264 974 April 2025  23