Surf PI - successful build

[Qiaozhi]

Email is the same, if you need it again, will send it off to you.

Sven - please can you send me an email? I sent you a PM yesterday about this. When I used the email address associated with your Geotech account, it bounced back as unknown.

[chrisw]

I havent fixed my led problem yet but I made a video today showing the depth test on my Crossbow; its an awesome detector board check it out;

http://youtu.be/wH4avCfVdHk

Chris

[SVEN1]

I started up on a second board, everything is working to the point I am at.
So I will have to take a look at the LED section.
Thanks George for the upcoming revised Gerber files so everyone has a new reference tool.
Email is the same, if you need it again, will send it off to you.

[chrisw]

Thanks for your investigatory time consuming research George it is appreciated. I will give the track alterations a look next.

Chris

[Qiaozhi]

Hi Ken,
I tried Georges idea of reversing the diode and connecting the cathode to the -5v. Right by the side of the diode connections in the pcb was a small hole which beleive it or not was connected to the -5v. So all I had to do was move the LED flying lead 3mm to the right and reverse the diode. So the mod only took 2 minutes. Unfortunately it still doesnt work; the LED is permanently on now and by varying the preset the LED just gets a bit brighter. So maybe I still need the track alteration.

The confusion here was caused by the LM311 having its inverting input on pin 3, and the non-inverting on pin 2; which is the other way round from other similar devices.
Also (when I suggested connecting the LED to -5V) I forgot that the LM311 has an open-collector output, so that's not going to work.
I'm afraid a cut-and-strap operation is the only alternative.

[chrisw]

Hi Ken,
I tried Georges idea of reversing the diode and connecting the cathode to the -5v. Right by the side of the diode connections in the pcb was a small hole which beleive it or not was connected to the -5v. So all I had to do was move the LED flying lead 3mm to the right and reverse the diode. So the mod only took 2 minutes. Unfortunately it still doesnt work; the LED is permanently on now and by varying the preset the LED just gets a bit brighter. So maybe I still need the track alteration.

[kenny444]

Hi regarding the track cutting, I would do this, fit the IC in an extra turned pin socket, leave out pins 2 & 3 solder some resistor legs in the the socket and cross them over, solder to IC legs. No cutting, no track repairs, in the past I have done strange things like bend all the legs of an IC the wrong way and fit it to the back of a PCB, because of little working room, sounds odd but it works.

[chrisw]

Thats great George thank you for sortin that for me. I had a funny feeling pins 2 and 3 were backwards, but my knowledge of electronics is limited so it was only a guess.
Thanks again
Chris

[Qiaozhi]

OK guys, I've located the problem. The schematic is correct, and the LED should turn off when the inverting terminal of the LM311 falls below the -2.5V reference voltage. However, in my notes for the REV-A version I wrote: "Note: Nat Semi 1994 LM311 datasheet has pins 2 and 3 back-to-front! Use ST datasheet as reference."

Apparently I forgot to correct that error when REV-B was created. Also, bear in mind that I've never built a REV-B version myself. Unfortunately you will have to cut-and strap the tracks (swap over pins 2 and 3) to fix the battery warning circuit. The only problem is that one of the tracks runs under U8.

As this is a simple fix, I'll generate a new set of Gerber files later and send to Sven.

Edit: Actually, there is one other [simpler] fix ... reverse the LED, and connect the cathode to -5V.

[chrisw]

Hi Sven, Sorry to hear about the missap, but thats interesting to hear you had the same problems.
Im wondering if the circuit is correct. Because I have used the correct parts, in the correct holes obviously. The rest of the board works perfectly so power supplies must all be ok. I even suspected the LED. I think I read George originally used a bigger LED which had a reverse breakdown voltage of 12v. My smaller LED has a reverse breakdown voltage of 5v so I thought maybe the voltage maybe goes a little high acros the LED and it conducts. I dont know if my theory is correct. But I stuck another LED in series to see if this made a difference but it doesnt. So it looks like I will have to leave the LED out because I am very pleased with the rest of the Crossbow.

[SVEN1]

Chris I had the same problem with the LED and I even adjusted the power supply voltage to see if that would make a difference. At that point I also grabbed the coarse power supply adjust knob instead of the fine adjust, well accidentily peaked the supply voltage and popped a few parts. OOPs.

[chrisw]

Hi guys thanks for the replies. I have all the voltages at you show Chet. I can set the LED to come on at 10.5v but it is always on over 10.5V and always off under 10.5V which is working backwards. I cant see how I would have wired anything backwards because it is all on the pcb and I just have two holes for the led. I feel if pins 2 & 3 were reversed then it would work the right way round. Everything else on the board works beautifully and nice and deep on coins.

Chris

[Qiaozhi]

Any Ideas especially George

Chris

I haven't looked at this design for about 4 years, but chrisw's advice is correct. You probably have the 1M trimmer adjusted to the wrong side of the threshold (junction of R45 and R46).

[Chet]

Hi Chris


I haven’t built the Crossbow yet but looking at the circuit I recommend the following checks;


1. Measure to make sure that the +5v and -5v are correct on pins 8 and 4.


2. Measure for approximately -2.5v on pin 3. This is ½ of the voltage between the -5v regulator and circuit ground. The -5v is divided equally by the two 100k resistors.


3. Measure for approximately -2.5v on pin 2 as you adjust the CERMET pot. The LED should go off and on as you adjust above and below -2.5v. The pot is dividing the voltage between -12v and ground (+12v) to set the trip point voltage near -2.5v.


I hope this is helpful,
Chet

[chrisw]

Crossbow Low voltage LED

I have another Crossbow question. The low voltage LED. I connected a panel mounted 5mm LED in the board today. It is a low voltage 2V led. I set it to come on at 10.5 as the calibration guide says, but then stays on all the way up to the full voltage. If I then turn off the power and then turn it back on gradually, when I reach the 10.5V the led comes on all the way to 12V again.

This is working backwards or am I looking at this all wrong? I thought it should come on from low voltage to 10.5 then turn off all the way to full voltage.

I have a series resistor in line with the diode to limit the current to 10mA.

Any Ideas especially George

Chris