The circuit of the starting stage of ЕМ-3 is a bit more complicated than for the subsequent stages, as it has some additional functions besides activation of the corresponding SCR:
1) Reception of signals from control unit, proximity sensor and sensor which controls the presense of projectile inside the barrel, with the shot being possible only provided all those signals are permissive.
2) Activation of IR-LEDs for optical sensors of the subsequent stages (their IN1 inputs are used).
4) Indication of all signals and states.
The difficuilty here is that different units sending the signals to the start stage, has different supply voltages. For example, the signal of "FIRE" and proximity sensor's signal are 5 V, but it needs 12 V to activate the 1st SCR and the subsequent stages' optics. The solution was to use the voltage level converter IC CD40109, which is directly intended for such situations. Besides, it can be set to Z-state (through EN inputs), which was used to further simplify the circuit.
The scheme of the control plate of the unit is depicted on fig. 1.

Fig. 1. Starting stage of  EM-3. DD1 - CD40109, T1 and Т2 - transistor array IRF7306.
S1 switch activates the optodetectors of EM-3 independently of presense of the "FIRE" signal. This is necessary to control the optosensors during adjustment of the circuit. As current is constantly  flowing through IR-LEDs when S1 is on, R4 resistor is added to limit it. Besides, 5 channel DIP-switch is mounted on PCB, which separately controls the sensors (by IN2 inputs), which is also convenient during adjustment.
Operation of the starting stage in cooperation with the sensors is  illustrated in video below: