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 Years ago I rescued an old, two-faced Simplex clock from a dumpster. It is a so-called slave clock: it has no time-keeping mechanism from itself but it receives a 24 volt pulse from a remote master clock once every minute.
Not having rescued the master clock I had to make an hour myself. The first version consisted of a 32.768 kHz chrystal, a 4060 binary counter, a 555 integrated circuit, and some assorted TTL ICs. The idea was to divide the 32.768 kHz time base from the chrystal by some factor and end up with one clock pulse per minute.
This version broke down after 16 years of good service: the rectifier, transformer and probably some other components burnt out for reasons I have not been able to discover. Unfortunately I lost the original schematic and I did not feel like reverse engineering it, so I had to make a new hour from scratch.
I wanted to do it differently this time: in stead of using the frequency of a chrystal, which is prone to temperature-induced drift, I chose to use the frequency of the mains power supply. This frequency can fluctuate in the short run, but the fluctuations cancel each other out over time, so over longer periods of time it is a stable 50 Hz. No need to adjust the clock but twice a year for summer- and wintertime.
The requirements for the clock electronics are:
- the clock anchor requires 24VDC to operate
- make one clock pulse of approx. 1 second duration, per minute
- all components must be cheap and readily available
This is the schematic (click to embiggen):
230 VAC from the mains is transformed down to approx. 24 VAC and rectified by a bridge cell (or 4x IN4002 or similar diode). Mind that the secondary voltage is expressed in VRMS - an average voltage and the voltage indicated by a volt meter. The peak-peak voltage, as measured by an oscilloscope, is √2 x VRMS, or approx 34 VAC.
A 7824 voltage regulator makes the 24 VDC for the clock's anchor and a 7805 voltage regulator makes the 5 VDC for the rest of the electronics.
The ICs used are CMOS, with an operating voltage of ~ 3 to 15 VDC so one could opt for a higher voltage as well (e.g. 12 VDC). Resistor values will have to be adapted in that case (notable the one for the LED), to limit currents to acceptable levels.
The unrectified secondary voltage is fed to resistor R1 and zener diode D1. Together they shape the alternating current to smnething that starts to look like a block form with a period of 20 msec:
The two inverters IC3A and IC3B act as a poor man's Schmitt trigger, shaping the signal to a square wave. The signal at pin P1 of IC4 looks like this:
It's not perfect but it's a good enough 50 Hz square wave for my purpose.
IC4 is a 4040, a 12 bit ripple counter. It counts the input pulses form the square wave and drives the outputs Q1-Q12 accordingly. The count advances as the input goes low (on the falling-edge of the square wave), this is indicated by the bar over the clock label (pin 10). Output Qn is the nth stage of the counter, representing 2n, for example Q4 is 24 = 16 (1/16 of clock frequency) and Q12 is 212 = 4096 (1/4096 of the input clock frequency).
In order to create the one pulse per minute for my clock I must divide the input frequency by 3000 (50 Hz = 50 pulses per second, times 60 seconds). Three thousand (decimal) is 101110111000 (binary), so I must use the outputs Q12, Q10, Q9, Q8, Q6, Q5 and Q4. All these outputs are AND-ed together so that at the output of IC6C we have the one pulse per minute.
Note that if you live in a country where 50 Hz is not the mains frequency, you'll have to use different outputs and AND them together differently, to obtain a once a minute clock signal.
The LED is there only for visual feedback, an indicator that the clock is actually running.
With the selector switch I can chose which signal goes to the clock 'driver' circuit: the one-pulse-per-minute described above, or a signal that changes much quicker, which can be used to fast-forward the clock to set the time.
The output of IC6C goes high once a minute and when it does it immediately resets the counters in IC4, which will start counting up from zero again for the next minute. This same pulse goed to inverter IC3C, so that the input signal for IC7, a 555 goes LOW once a minute.
The 555 is configured as a monostable multivibrator. This is also known as the delay mode of operation. It functions as the driver stage for the clock. When the signal on the trigger input ('TR') goes low the output signal on 'Q' goes high for a period of time that is determined by the formula:
t = 1.1 * R4 * C5
For the given values of R4 and C5, the time that Q goes high is about 1 second which is enough to reliably drive the clock's anchors.
The 555 output switches on transistor Q1, which applies the power to the clocks's anchors, symbolized by the coil symbol L2. A quenching/suppression diode D2 prevents premature failure of Q1 by the reverse voltage when power is removed from L1.
Note: When using CMOS components like I did here, it is important to tie all unused inputs to a fixed voltage like I did for IC3D, IC3E, IC3F and IC6D. This to prevent random switching of the unused ports due to undefined voltage levels. Because CMOS dissipates power only when switching, leaving them open will result in energy waste.
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