This diode is needed as the supply voltage used by the processor is lower than the supply voltage of the receiver (due to D1). Without D2, the input signal voltage could exceed the supply voltage of the processor, which may cause the processor to mal-function. D2 is a signal diode, therefore the voltage drop across it is much lower than an ordinary diode. A signal diode is necessary to ensure that the low level voltage at the input pin to the processor meets the specification.
The component RES is a three terminal ceramic resonator which provides the clock and timing reference for the processor. It provides a frequency stability of +/- 0.5%, making this the accuracy of the timing measurements performed by the processor. However, the processing of the input pulse is more accurate, as the same timing reference is used for both the input measurement and the output pulse. The output pulse width is therefore very close to the input pulse width.
The remainder of the circuit is simply the five option links used to configure the operation of the program. The processor provides internal pull up resistors, so the option links connect these input pins to ground.

Construction.
The complete circuit can be fitted on a small piece of stripboard, 13 holes by 11 holes with the strips running lengthways. First, cut the strips, where indicated in figure 2, using a spot face cutter, or a drill bit. This shows the board with the copper strips facing. After cutting the strips, clean off any swarf and carefully inspect the cuts to ensure they have completely broken through the copper tracks.
Next, turn the board over, taking care to keep row 1 at the top. Now, add wire links to the board where shown in figure 3, noting the two special cases described below.

 Use solid, tinned copper wire for these links. Component leg offcuts are often useful here.
There are two special cases. Firstly a link which is bent to avoid other components. This link starts at J7, goes diagonally over the top of I8, and is connected at I11. The second special case is the link which provides the ground connection for four of the option links. This starts at B3, is bent around the link between A3 and A5, passes through the board at A7, and then travels on the copper side of the board (shown dotted in figure 3) connecting to each of A8, A9, A10, and A11. This link, and the link from A3 to A5 are the same signal so do not need to be insulated from each other. If a pack of switches is to be fitted instead of using wire links, do not solder this link to positions A8 to A11 yet, as these holes are needed for the switch connections.
Now, all the actual components may be added. Their positions are shown in figure 4. Start by fitting the two diodes being careful to orient them correctly. Next fit the capacitor, the positive lead is usually longer than the negative lead, followed by resistor R1. Note how one of the leads of R1 is bent to connect at I3. Next fit the resonator (RES) which does not matter which way round it is fitted.
The wire links are shown to help position the components. Care should be taken with IC1 and Q1 as these are sensitive to static electricity. Q1 is fitted so that when it is folded flat, the metal tab is uppermost, and fits over the top of IC1. I found it easiest to bend the legs of Q1, and check its fit, before soldering it in place. If fitting a set of dual in line switches instead of wire links for OP1-4, then these are fitted through holes D8-11 and A8-11, soldering the wire link at the same time. Wire links for the five options OP1-OP5 may be fitted later when the function is being defined. .

Figure 2. Figure 3. Figure 4.

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