This circuit works in exactly the same way as my original circuit - i.e. the existing manual switch is used to 'reset' the circuit once automatically locked out as well as engage or disengage OD manually. The LED shows when OD has been locked-out i.e. whenever the manual switch is closed but the gearbox switch is open.
This circuit has been bench-tested, but needs to be tested on-car to check for any unwanted operations. Cars of our era are very 'noisy' electrically with many spikes and random pulses as things are turned on and off, as I have found with purely electronic circuits in the past, and a commercial version of this suffers from this problem so I've been told. It's not something I've ever encountered with my original circuit, quite possibly the brief (in our timescales) time that a relay needs to operate and release is able to ignore microscopic (in time and voltage) hiccups in the signal from the electronics.
The characteristics of a thyristor are such that once a signal has been connected to the gate 'g' and turned the thyristor 'on', which allows current to pass from the anode 'a' to the cathode 'c', the gate signal can be removed and the thyristor will continue to conduct until something else interrupts the current. This can be the manual switch (to turn OD off manually), or the gearbox switch opening when changing out of an OD gear, or turning the ignition off.
The capacitor enables this to be achieved solely with the original manual switch, and not need two other push-buttons that have to be mounted somewhere, as other systems need. The capacitor supplies a brief pulse of +ve voltage to the gate as it charges up when the manual switch is turned on, and this is enough to turn the thyristor on. The 10k resistor in parallel with the capacitor ensures that the capacitor can discharge when the manual switch is turned off, as if the capacitor is still charged when turning the manual switch on again it will not generate a pulse to turn the thyristor back on again. Note that opening and closing of the gearbox switch will not re-engage OD as this does not generate a pulse to the gate, only turning the manual switch on (or off and back on) will do that.
R1 and R2 form a potential divider to give a reduced voltage to the gate - in the order of 2v or so - as required by the specification of the device. This is despite the thyristor itself being rated at 650V and 13A, which is deliberately way over-kill for the 12V and 1A of the OD.
The LED will glow when OD is locked out i.e. the manual switch is on, the gearbox is in an OD gear and that switch is closed, but the thyristor is not triggered. The LED 'sees' 12v from the manual switch, and an earth via the gearbox switch and solenoid, and as the solenoid resistance is only 15 ohms that is more than enough for the LED.
The LED can be changed to act as an 'OD engaged' indicator by moving its +ve connection to the thyristor cathode and its -ve connection to earth.