Mars Electronics was a big player in the coinchanger industry and it developed a new coinchanger that would be ready for new coin denominations such as the Euro.
The really important innovation in this series was that it could calculate how much cash was in the machine at any time so that sales could be audited.
I worked in a large engineering design team, each of whom was designing a specific part of the whole system. My role was to design the Acoustic Module. The coins in the machine are located in 5 vertical tubes beneath this module. The height of each stack of coins is measured by initiating a loud noise with an easily defined spike, routing the sound wave to a port above each coin stack and measuring the time taken for the sound wave to bounce off the top coin in each tube and return to the ports. This is very tricky to do in practise.
The sound was generated by firing a spark across two electrodes and routing the sound wave in closed tunnels to the top of each stack. Tiny microphones were located at the top of each stack and the time lapse was recorded for each stack. After much prototyping the system eventually worked.
I designed the module as two halves of an injection moulded clam shell with areas designated for electronics, PCBs,wiring looms and microphones.