Toltec 100 VCO Mod

[Merc]

This is part of a project I'm working on that's based on the Toltec 100. This will add tone ID in discrimination mode and VCO pinpointing in all metal mode. I have it built on a breadboard and have it hooked up to my Toltec 100 and so far it seems to work fine. I have built a PCB layout(SMD) and have all the parts I need except for the CD4053 which I ordered the wrong case style. Never tried working with SMD so this will be a learning experience.

Merc

[porkluvr]

Hello, Merc. Unless you have a halo-lamp or binocular microscope, you should stay with 1206 or larger size R-C components as much as possible. It looks like that's what you're doing so you should be OK.

Even still, you'll need a head-magnifier at a minimum before you should tackle SMD. It's good to not have a bunch of holes to drill, but SMD can be a headache without proper tools. (Ask me how I know.)

The Toltek looks like a daunting project, especially considering that the RC4200 multiplier chip is extremely rare nowadays. Did you find an inexpensive source for RC4200 or do you have a work-around up your sleeve?

Also, I'm curious about your VCO input range. Won't that be a bi-polar signal coming out of the analog switch; so why wouldn't you use a bi-polar power supply?

Good work on your PCB.
I tried KiCad many years ago but could not even get a toe-hold to get started. I was stymied, so I'm stuck using a "free" version of Eagle.

[Merc]

I am planning to stick to 1206 size SMD as much as I can. I have a magnified light to use and my closeup vision is still OK. I got three RC4200AM SMD off ebay for about 4 bucks each. I just bought a Toltec II an noticed it also uses an SMD RC4200AM which makes me think tht Toltec II is not too far off from a Toltec 100.

I have used Kicad for a while now. I just couldn't figure it out for a while and then I found a simple tutorial online and that was the breakthrough I needed. I use it under Linux and Windows. My main PC for PCB work is a Linux box with dual 20inch monitors. In Kicad, I can have the schematic open in the left window, the PCB layout in the right window. When I click on a component in either window, the other program centers the screen and crosshair on that same component.

Merc

[Merc]

The VCO input will always be positive I believe. I am basically using the meter signal and the meter does not go negative. The circuit has a minimum frequency set by a divider network. I had to do this because the osciillator would not run with 0V input voltage. I also wanted to limit how low the VCO would run. I need to measure the frequencies but just going by sound it works great as is. I get a nice high tone on quarters, a much lower tone on nickels and foil is below it enough that it's easy to tell the difference.

Merc

[Merc]

http://www.youtube.com/watch?v=WtWE6...e_gdata_player

This is a link to a video of me demonstrating the mod.

[Merc]

I just found a mistake in the schematic. I didn't notice it until I built the board today and installed it. It did not work right and I finally found that I had left a resistor out. I have added R12 on this schematic. I have the mod installed and hope to try it tomorrow.

Merc

[Merc]

Tested this mod in the field today and it worked great. Found a wheat penny at about 6 inches and got a nice high tone. I find this mod most useful in trashy areas. All the tones can get overwhelming but the high tone of coins gets your attention and you don't have to watch the meter constantly.

Merc

P.S.
I'm attaching a corrected layout with the added resistor.