![]() Note that at count 9, the clock to the second flip-flop is masked by the logic causing state 9 to transition to state 0, rather than state 10. Figure 5 shows four flip-flops, the logic, and the internal clocks driving each flip-flop to make a divide-by-10 counter. To make it count from 0 to 9, it needs some steering logic on the flip-flop toggle inputs. A four flip-flop counter naturally counts from 0 to 15. Counters made by a chain of n flip-flops result in binary ripple counters capable of dividing by 2n. The clock is made of a collection of counters. The two diodes connecting bases to the input allow a short negative-going pulse to “steal” the base current for a moment on the falling edge of the input, causing the flip-flop to toggle. In other words, it has “toggled.”Ĭonsider Figure 3 again. The flip-flop has “flipped” to the other state. Q2 will turn off, output B will start to be pulled high by R4, and current A will start to flow through R3 and C2. Imagine that current B is somehow interrupted for a moment. Output A is high current B is flowing into the base of Q2 so Q2 is on and output B is low. It’s Toggle Time!Īssume the previous condition with Q1 off and Q2 on. Note that they both cannot be on and they both cannot be off. ![]() The other stable condition is Q1 on and Q2 off. No current flows through R3 (current A) so Q1 is off (which is the initial assumed state). A current flows through R2 (current B) into the base of Q2 switching Q2 “on” so Q2 conducts and pulls output B to ground. 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. After decades of seeing projects and circuits using ever increasingly complex integrated circuits, I yearn for simpler times.
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