Can silverdog's kit be made to run on 9 volts?

[morganton]

Hi,

I put silverdog's excellent kit together, but I can only get it to operate on 12 volts. I would like to be able to use 6 x 1.5V AA's (for nominal 9 volts) if possible.
A 6 x 1.5V AA battery pack fits nicely in the enclosure I'm using.

Are there parts substitutions/modifications that will allow it to operate on 9 volts?

I have made one modification to his kit, substituting a TIP127, but it did not affect the required voltage.

Silverdog's kit does work great on 12 volts.

[satdaveuk]

What you need to do is look at the original Barracuda circuit and alter accordantly although from the top of my head the Darlington transistor was the main reason, Remember once deviating from using the FET you will need to reset your offset etc, its not just easey as substitution, once done they work fine on 9V.

[morganton]

Thanks Dave,

I set pin 6 on the 5534 to zero volts after substituting the TIP127. I'll go through the original schematic and see if I can figure out what else is may have been changed to make it now require 12 volts instead of 9 or 9.6 volts.

[Tepco]

First you need to replace 78L05\79L05 regulators with low dropout types, with 6 standard AA cells voltage can drop below 8V at the end of battery lifetime. Standard regulators will lose regulation, some 2.5V difference between IN\OUT is needed +7660 loss. Also to increase pulse width to keep same coil peak current. If space available inside enclosure is limited, better use 10 AAA size rechargeable NiMh batteries, will be cheaper on long run.

[morganton]

First you need to replace 78L05\79L05 regulators with low dropout types, with 6 standard AA cells voltage can drop below 8V at the end of battery lifetime. Standard regulators will lose regulation, some 2.5V difference between IN\OUT is needed +7660 loss. Also to increase pulse width to keep same coil peak current. If space available inside enclosure is limited, better use 10 AAA size rechargeable NiMh batteries, will be cheaper on long run.

That's a good idea about the LDO regulators. I did a quick check with Digikey--for 78L05 looks like Microchip's MCP1702 would work (330mV drop-out at 250mA). Not a great deal to choose from for 79L05's, but maybe TI's LM2900T-5.0 could be squished on the board (.6V drop-out at 1 amp, but only in a TO-220-3 form).

[morganton]

Well, I was able to get my silverdog kit to run from my 6 x 1.5V AA battery pack (9V) by changing out the 7660S with another one I had in my inventory. I was going through the circuit and noticed that the output from pin 5 on the original 7660S that came with the kit was only -3.26 volts with my 9V battery pack, but was -11.66 when using 12V as a power source. When I replaced the 7660S with another unit from my inventory, the voltage on pin 5 came up to -8.75V and the detector appears to now work fine with my 9 volt battery pack. This might be something that other folks could try that were also having problems getting it to run on 9 volts.

How long it will operate on 9 volts is now the question, but at least it's working now and I might be able to use my 6 X 1.5V AA battery pack after all.

I do think that Tepco's suggestion to use LDO regulators as noted above is a good idea and I'm going to change them out as soon as I can order some.

[Koala]

Be careful with the specs of the 7660S some with only "S" at the end are only rated to 10V it's advisable to use a 7660A/SC which will give a few volts extra headroom. Seems to depend on the manufacture as max voltage are listed differently on different sites

[morganton]

Also noticed that according to this 7660S data sheet, http://www.digchip.com/datasheets/pa.../7660S-pdf.php page 6, the 220 uF electrolytic connected to pin 5 "must" to be connected with negative to pin 5 and positive to ground. The PCB has the positive lead to B+ and negative to pin 5. Don't know what difference, if any connecting it according to the data sheet would make . . .

[Qiaozhi]

Also noticed that according to this 7660S data sheet, http://www.digchip.com/datasheets/pa.../7660S-pdf.php page 6, the 220 uF electrolytic connected to pin 5 "must" to be connected with negative to pin 5 and positive to ground. The PCB has the positive lead to B+ and negative to pin 5. Don't know what difference, if any connecting it according to the data sheet would make . . .

On Silverdog's site there is a Surf-PI schematic, and it does show the 220uF cap with its positive terminal connected to ground. However, you are correct (and no-one seems to have noticed this before) on the PCB the positive terminal is connected to the battery +ve input. However, I think in practice this will not cause a problem.
The 7660 devices can be a bit touchy as they tend to drop out below about 8V, and there are two different types with different ranges. Anyway, the Surf-PI is designed to work from a 12V supply.

[Koala]

Also noticed that according to this 7660S data sheet, http://www.digchip.com/datasheets/pa.../7660S-pdf.php page 6, the 220 uF electrolytic connected to pin 5 "must" to be connected with negative to pin 5 and positive to ground. The PCB has the positive lead to B+ and negative to pin 5. Don't know what difference, if any connecting it according to the data sheet would make . . .

Don't have a very good schematics but I don't thing the cap you are talking about is going to the ground but is going to battery plus

[morganton]

On Silverdog's site there is a Surf-PI schematic, and it does show the 220uF cap with its positive terminal connected to ground. However, you are correct (and no-one seems to have noticed this before) on the PCB the positive terminal is connected to the battery +ve input. However, I think in practice this will not cause a problem.
The 7660 devices can be a bit touchy as they tend to drop out below about 8V, and there are two different types with different ranges. Anyway, the Surf-PI is designed to work from a 12V supply.

Just to satisfy my curiosity, I rerouted the 220 uF cap connected to pin 5 in accordance with the data sheet. It's an easy task because there is a via next to the 7660 IC (in back of the DIP socket close to pin 4 and 5) connected to ground which can be used to solder the positive lead of the 220 uF cap.

The role of the 220 uF cap (C2 in the paragraph below) regarding the operation of the 7660 is explained by the data sheet as follows:

The mode of operation of
the device (the 7660) may be best understood by considering Figure
12, which shows an idealized negative voltage converter.
Capacitor C1 is charged to a voltage, V+, for the half cycle
when switches S1 and S3 are closed. (Note: Switches S2
and S4 are open during this half cycle.) During the second
half cycle of operation, switches S2 and S4 are closed, with
S1 and S3 open, thereby shifting capacitor C1 negatively by
V+ volts. Charge is then transferred from C1 to C2 such that
the voltage on C2 is exactly V+, assuming ideal switches and
no load on C2.

I guess there is always the possibility that the PCB routing of the 220 uF cap may have been causing the 7660 to have incorrect output values on pin 5 (see post 6 above).

I'm still fabricating the stem and other parts of the detector, so I haven't had a chance to test it fully.

[Qiaozhi]

Just to satisfy my curiosity, I rerouted the 220 uF cap connected to pin 5 in accordance with the data sheet. It's an easy task because there is a via next to the 7660 IC (in back of the DIP socket close to pin 4 and 5) connected to ground which can be used to solder the positive lead of the 220 uF cap.

So ... was there any difference?

[morganton]

Sorry, I've been distracted with other stuff this afternoon. I turned it on for just a second without a coil and it seemed to work about the same as far as the opening beep.

I plan to spend some time tonight testing it and I'll post the results.

[morganton]

Ok, so I had a chance to test my partially completed Barracuda with my unfinished coil and it seems to operate the same as before I re-routed the 220 uF cap. I am running it on my 9 volt battery pack. It detects a U.S. quarter at about 8 inches.

I have attached a photo showing the via where I soldered the cap's positive terminal.