Vending electronics

Possible choice would be 2 off 8 bit PLCs  driving a matrix of solenoid locks ,each securing a separate cublcle. The Door would be held shut by the lock and a spring would open the door. An open door would signify "out of stock"

Both  plc's would be  programmed to float outputs on non-selection with one driving a selected output +ve and the second -ve on selection, .The combination of outputs would drive only 1 solenoid  -  intrinsically fail secure as with no power - the fail situation - the solenoids would not unlock.

If I was starting from scratch I would opt for an 8 bit microcontroller - my favorite bit of kit  - same matrix idea but able to be programmed to lock out any previous selection. I would select a 12 volt supply and the locking mechanisms would be based around bog standard car door locking motors - availalble from any scrap yard for next to nothing and once unlocked a "fill" command could reverse matrix voltage driving normally low signal high and normally  high signal low - thus reversing the motors back to the "lock" position.as the cubicles are filled

As to the supply - an obvious choice would be a desktop computer power pack - ATX style - easy to mod to give motor drive voltage - 12v and microcontroller supply - either 3.3 or 5 volts depending on the choice of micro.

As to handling payments it shouldn't be difficult to modify a commercial card payment machine as used to validate and pay by credit card in a shop to activate the output matrix after a valid selection is made and - assuming the product is expensive enough to justify loading with the cost of it let the card company take care of validating the card and accepting payment. It could prove a bit of a pig to design a card reader/writer and prove it's accuracy. Most of your clients would be unhappy about the thought of you having the ability to take money from their card with an unproven device.

Paul,  thanks for the detailed answer, it provides me with many practical options that I did not consider.  I appreciate it and will start researching and testing some of the points you addressed

A thought re the power pack. the ATX or the ATX2 packs - with or without -5v not now used - are very easy to mod to power up on application of power. Simply take the switching wire low. However I have played with a few, making power packs for all sorts of bits and pieces and have found the if connected to the supply with the power output active they can tend to go bang. My experience is that a short while is needed before switching on the output or the start surge kills the main diodes . I find that if you use a normally closed relay with the contacts wired across "power on" and 0 volts and the coil wired between 0 volts and the auxiliary supply with an electrolytic of 500uf or so in series when switching on the coil is activated by the current through the cap and pulls in. deactivating the supply. After a short while - millisecs to seconds depending on the cap value and coil resistance the cap[ charges - the current drops to zilch and the relay drops out - activating the supply. Thus this delays switch on and reduces instantanious surge,

Depending on the current in the non-vending phase of operation you might have to introduce a bit of load to stabilise the pack - put lots of pretty lights on the machine as an example. Pull 500 ma to an amp and the pack will be happier. This is one time when you have power to waste.

Also if you use a credit card company for collecting the  money you will have to have a phone line near the machine.

Thanks!  I have enough to get me farther down the road.  Appreciate the assistance

Thanks!  I have enough to get me farther down the road.  Appreciate the assistance