Crystal controlled 1 pulse per second clock.

Allan Rydberg

This easy to make circuit will provide pulses at a highly accurate rate of one per second (1 Hz).

The circuit is based on a common quartz clock movement that seems to be in most of today's wall clocks. A suitable movement can be salvaged from a clock or purchased new from a hobby shop.

Figure 1. Common quartz clock movement.

The movement I used is shown in Fig. 1. It has been disassembled, and most of what is shown may be discarded. The important part of the movement is the circuit board and motor assembly shown in the bottom left corner of Fig. 1. Note that in disassembling the clock movement it is important to follow the connection to the positive battery terminal and to mark the positive terminal on the circuit board.

The clock signal is derived from a crystal-controlled oscillator. The circuit divides the signal into two alternate signals that each pulse every two seconds. We will call these the odd and even pulses.

These signals are then fed to the two ends of a coil where the center tap is connected to ground. Thus, on all the odd pulses the magnet is pulsed on in a north-south configuration and on all the even pulses the magnet is pulsed on in a south-north configuration. This is what turns the magnetized armature and drives the clock.

For our purposes we will power the clock with the usual 1.5 volt AA battery and use 2 signal diodes to pick off the two signals going to the electromagnet. Then we will "and" these two signals together with 2 diodes. I used 1N4148 signal diodes.

This will provide us with a 0 to 1.2 volt pulse every second. The pulse width is about 30 milliseconds.

There are many ways to convert the output to a TTL pulse. I used a LM339 quad comparator, and the schematic of my circuit is shown in Fig. 2.

Figure 2. Schematic of a simple circuit that transforms the pulses from the clock into TTL-level pulses.

Figure 3 shows the circuit board which produces a TTL pulse.

Figure 3. The circuit board shown here is the hardware version of the schematic in Fig. 2.

Figure 4 shows the pulse on an oscilloscope.

Figure 4. The completed circuit provides pulses with a duration of about 30 milliseconds.