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Old 17th Jun 2017, 11:00 am   #17
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: FM to AM Converter?

Congratulations on John as the first forum member to 'go public' on the successful completion of the project. Good to know that it's working. John has kindly donated the components to me to make the board, and I've obtained the programmed pic from the author.

Given that it uses 20mA, I wonder why an on/off switch wasn't included, given that the unit won't switch off until 2 hours has elapsed, which might be long after the desired listening period, and needlessly drain the batteries? I wonder if there's any detriment in including an on/off switch?

The FM module 'U2' on the strip-board layout, doesn't actually have the same 0.1"/2.5mm spacing are strip-board or ICs - the five notches span only 8mm rather than 10mm, so as John says, it needs to be mounted on fine wire. Luckily, they're so cheap at £2.19 for two post free from a UK supplier on e-bay, so you get one to mess up with, and one to succeed, hopefully! It's evocative of the old poem about sowing corn: "Four seeds in a row - one for the rook, one for the crow, one to die and one to grow".

I'm not a fan of strip-board, albeit for a simple layout such as this it has some merit and was the method used by the designer. I only discovered the odd spacing of the notches when I set about designing a PCB for the converter, which is at an early stage. Initially, I spaced the notches at 2.5mm then was puzzled to see that the module didn't line up, so I had to adjust the spacing accordingly. I'll mount the module on 1mm veropins. I'm not keen on the board being secured on only two bolts at one each end of one edge of the board and 'swinging free' at the other edge, so I'll be mounting the PCD on four 6BA c/s screws, one at each corner.

The circuit shows no connection to pin 4 of U1, yet on the strip-board layout, there is a wire connected to pin 4, the other end of the wire going to a strip which has no other connections. Seems odd!

John has shown that if built as the strip-board layout shows, the project works fine. Whether or not my PCB layout will work - only time will tell when I've finalised it and built it. I've only done a component overlay at this stage of development, with an 'X-Ray' of the underside track layout. Not thoroughly checked, so it's only attached here for interest - not as a 'good to go' layout.

The second pic shows the FM module - a poor pic, but the solder pad notches can be seen on the right hand side, and from what John says, they're easily dislodged if great care isn't taken.

Who's going to be the next past the finishing line?!!
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Name:	FM AM Converter Circuit  PCB Draft V2 Updated.jpg
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Name:	FM Module.jpg
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