![]() The collector of Q1 is high impedance so Output A is pulled high by R1. The bi-stable circuit in Figure 4 will be used to explain the operation of the toggle flip-flop.Īssume that transistor Q1 is in the off state. The heart of this clock is the two transistor toggle flip-flop shown in Figure 3. A divide-by-12 counter completes the clock by showing hours. Following that is another pair of counters: ÷6 and ÷10, showing minutes and tens-of-minutes. The high bit of this counter drives the next counter in the chain: a divide-by-six counter showing tens-of-seconds. The four bits of the counter are decoded into a one-of-10 signal that drives a seven-segment display showing seconds. The 1 Hz - which is also a one second per pulse clock - drives a divide-by-10 counter who’s output is 10 seconds per pulse. A 2 Hz clock is also routed to the time setting switches. Next, a prescaler divides the 60 Hz by 10 and six, resulting in a 1 Hz clock. The power supply (lower left) rectifies and filters the incoming nine volts AC, converting it to nine volts DC to operate the circuitry and a 60 Hz clock. The Big Pictureįigure 2 shows the design at the functional block level. This article will explain the circuitry at both the logic level and the transistor level. After a few years of “work” (it felt more like play), the final parts count is 194 transistors, 566 diodes, 400 resistors, and 87 capacitors. To return to those glory days, I decided to build a digital clock using only transistors as the active elements. ![]() These circuits were from dusty hobby books found at my local library with names like “29 transistor circuits” or “electronic hobby circuits.” Transistor Clock My flashlight controlled relay could control a buzzer music from my cassette tape player played on my radio with a two transistor circuit my amplifier could drive a speaker. As a teenager, I built fascinating and wondrous circuits using just a few transistors. By all means try it and find the problems otherwise you won't learn and then come back and read this again.After decades of seeing projects and circuits using ever increasingly complex integrated circuits, I yearn for simpler times. It will be better to have the flip flop set synchronously on the same clock. ![]() You have to watch which edge of a clock is the active one and propagation delays otherwise you will get spikes through the gate and false triggers. Having an asynchronous flip flop controlled by a clocked divider and then gating the same clocks is going to have race conditions. When people first try building logic circuits they tend to think of it as static and instant and then run into problems with clocks. The diagram you drew invokes some hidden problems. There are enough keywords above to keep you busy on Google for a few days. The other pitfall is noise immunity, input signals need to be processed, usually with a Schmitt trigger and manual switches need to be debounced. The solution is to treat all logic I/O as analogue signals at the jacks and use simple transistor and diode circuits to buffer the logic. Just connecting a negative analogue voltage to a logic IC will destroy it. It is also not a good idea to have a limited voltage range logic signal on the same connector as wider range analogue signals. If you take them to the outside world through resistors they will combine with random amounts of capacitance in patch cables to slew the signals and they become attenuated which will create problems, especially with triggers. They are designed to have fast rise times and the inputs expect that. In general it is not a good idea to take logic signals off a pcb. Could you suggest a simple schematic for the output resistor?
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