Olympus Trip 35 Frame Counter

The Olympus Trip 35 frame counter is a clockwork mechanism, which displays the frame number that is in the chamber. The counter advances forward when the user rotates the film advance thumbwheel; however, opening the film compartment door will automatically reset the counter.
The counter will count to a maximum of 36, which is the number of frames in a 35 mm film roll. This mechanism is a great bit of engineering because it is spring loaded and resets automatically. All the high quality manual cameras of that period had such a mechanism, and to remove the film counter there are just two screws as indicated in the photograph.
Thumbwheel Powered

As you can see, there is a cogwheel, which connects the film advance thumbwheel to the counter mechanism. The thumbwheel forwards the count in the counter mechanism and stores energy in a small spring by stretching it. The energy stored in the spring is later used to automatically reset the counter.
Counter Mechanism

The counter mechanism was a very reliable piece of engineering, I have never heard of it ever failing, and I have opened it up just out of curiosity to see how it works, as I am sure it might interest other engineers out there.
Reset Spring

On each count, the spring stretches further and locks in that place. When the count reaches the maximum number, it triggers a reset condition, and the energy stored in the spring powers a smaller cogwheel, which rotates the disk back to the zero frame position.