PICKINI - an easy to build PI detector

[DIVER]

Many thanks Bernard for your help!

[satdaveuk]

The BS170 is just a level translator/inverter from 5V to 12V for the MOSFET gate drive pulse - 100microseconds wide.
In theory any NPN transistor can do this - no idea if this influences the switching speed. I guess only the switch off speed is critical ?
I haven't tried it with any other transistors

Best regards,
- Bernard

Thanks Bernard
You seem to have this project well covered
Good job
Many thanks

[Dixix]

I have the circuit "PICKINI" built. Finally, a circuit that works!!! Thank you Bernard for publication. Also, the HEX-files could be easily loaded into the 16F1824. It is fun to see how it works. And gives courage to build the circuit with 16F616 and 20MHz Xtal. I have two short questions: 1 Have I understood correctly that the multi-turn pot (R21) is for adjusting pin 3 of the opamp (8V)? 2 The shielding. Do I have the shielding tape as shown in Figure 1 to make the coil, or as shown in picture 2? Thank you, Bernard, for this nice project! Regards, Dirk

[F117]

Have I understood correctly that the multi-turn pot (R21) is for adjusting pin 3 of the opamp (8V)? 2 The shielding. Do I have the shielding tape as shown in Figure 1 to make the coil, or as shown in picture 2? Thank you, Bernard, for this nice project! Regards, Dirk

Dirk,
1. The multiturn potmeter is indeed for adjusting the DC offset of the opamp: 8.0V at pin 3.
2. I normally use shielded microphone cable = ( 2 conductors + common shield ) instead of coax (1 conductor + shield ?).
The connection in picture 1 is wrong IMHO - but then again I'm not a coil expert. In picture 1 you will short the shielding = GND with one of the coil connections.
So, use picture 2 -- but what is the outer shielding of your coax cable then ?? Does it have inner shielding = one of the coil connections AND outer shielding = GND (shield) ??

Best regards,
- Bernard

[Dixix]

Sorry, I was blind .... In your circuit, a ground terminal is provided. Directly under the large capacitor. I'm going to get me today microphone cable.
Thanks for the note.
regards
Dirk

[Toros]

Hi F117,
I listen to the sound of the detector and I hear crackling. It looks very much like DC, which capacitor 10 uF can not filter and run to the speaker. It is possible that the sound is embedded in the CPU program. I work with PIC 16F616 and I wonder is there a way to eliminate this pop.
Is it the same sound in version 2, which uses PIC 16F1824.

[F117]

Hi F117,
I listen to the sound of the detector and I hear crackling. It looks very much like DC, which capacitor 10 uF can not filter and run to the speaker. It is possible that the sound is embedded in the CPU program. I work with PIC 16F616 and I wonder is there a way to eliminate this pop.
Is it the same sound in version 2, which uses PIC 16F1824.

Hi Toros,
The only sounds generated by the microcontroller are all listed here:
http://users.telenet.be/willaert/MD/...technical.html

They are the same with the 16F1824.

If you hear "crackling" : check the polarity of C15 = 10 microF coupling capacitor to the audio output.
Check any noise on the 5V power supply to the microcontroller.

Can you hear the normal 1Hz tick ?

Regards,
- Bernard

[Toros]

Hello, F117,
I use google translator and the word "cracking" may not be the most accurate.
Yes the sound is what is heard of http://users.telenet.be/willaert/MD/...technical.html. In headphones the sound can be heard clearly and distinctly and slightly irritating. To avoid that I installed additional gain stage and uses the speaker. The sound is heard again, but not as annoying. This is IMHO.

If I have understood that sound is embedded in the program and can not be changed. So I will stop asking questions about the sound.

I did the PCB version 1 and 2. Both boards are with processors 16F616. At low values of sensitivity potentiometer no problems. Consumption is around 55-58 milliamps. However, if the sensitivity is increased above average position of the potentiometer / over 12 hours / then repent consumption increases to 88-100 mA and 7812 begins to heat up. Is this normal. I ask this because at maximum sensitivity for detecting the distance detection to significantly increase and is close to 30-35 cm for 1 euro coin.

[F117]

Hi Toros,

Personally I prefer the sound ticks over complete silence. Detecting in complete silence can be very ennoying because you are never sure whether your detector is still operational or there is just nothing under the coil...
You can decrease the sound level by increasing the value of the 2 resistors in series with the audio output. Or by using real cheap, bad headphones...

At low values of sensitivity potentiometer no problems. Consumption is around 55-58 milliamps. However, if the sensitivity is increased above average position of the potentiometer / over 12 hours / then repent consumption increases to 88-100 mA

Again: the answer to this mystery is in the technical description here:
http://users.telenet.be/willaert/MD/...technical.html

When the sensitivity potmeter is turned up, the software will accumulate more samples. In order to have enough samples in a reasonable time and to increase the responsiveness of the detector, the pulse frequency is doubled - resulting in higher power consumption.
The LM2940-12 V regulator will heat up - depending on your battery voltage. If your batteries are 18 V - there is a 6V drop over the regulator. With a current consumption of 135 mA, this is about 800 mW dissipation in the regulator. This should still be acceptable without heat sink. As your battery voltage drops, heat dissipation also decreases.

I ask this because at maximum sensitivity for detecting the distance detection to significantly increase and is close to 30-35 cm for 1 euro coin.

.
You must have a very good coil then The best coins to detect with pulse induction are the 5 eurocent coins, as they are iron with a thin copper layer. That is why you find these coins left over by detectorists using high end discrimination detectors...

Regards,
- Bernard

[Toros]

Thank you F117,
Obviously, the detector must not run at full sensitivity.

[Dixix]

Hello,
I use 4 x 9V ACCU. The total voltage is 21 V.
The 12 volt regulator is very warm. Therefore, I use a small heatsink.


For the circuit I made a casing made of thin aluminum sheet. I have found that there is a very good protection against interference. So far, I could do without the shielding coil. Let's see what the result field tests ....

[F117]

Dirk,

I use 4 x 9V ACCU. The total voltage is 21 V.

How do you connect 4 x 9 V blocks: 2 in series = 18 V and then 2 of these in parallel (hopefully with 2 diodes in series ? ).
Or just 4 of these in series = 36 V ??

Also, the capacity of these 9V blocks is very low ( around 300 mAH ). NiMH batteries in series will give you a resolution of 1.2V and have a capacity of 2000-3000 mAH.
A simple constant current charger can be found here:
http://users.telenet.be/willaert/MD/...n/battery.html

I strongly recommend using 15x rechargeable NiMH AA cells in series instead of 9V blocks.

Regards,
- Bernard

[F117]

A quick calculation of the dissipated power vs temperature rise in the LM2940-12.
Based on this datasheet: http://www.ti.com/lit/ds/symlink/lm2940-n.pdf -- page 15

Power dissipation in the device = ( Vin - Vout ) * I => (18 - 12) * 0.135 = 0.81 W
Max juction temp = 125C - ( max ambient = 50C ) = 75C
Thermal resistance = 75 / 0.81 = 92.6 C/W

From the datasheet:
If the maximum allowable value for θ(JA) is found to be ≥ 53°C/W for the TO-220 package,
≥ 80°C/W for the DDPAK/TO-263 package, or ≥ 174°C/W for the SOT-223 package, no
heatsink is needed since the package alone will dissipate enough heat to satisfy these
requirements.
If the calculated value for θ(JA)falls below these limits, a heatsink is required.

So, in this case, the 12V regulator in TO220 housing can be safely operated without heatsink.

[Dixix]

Hello Bernard
Thank you for the very important information! Yes, I use 2x2 9V batteries. Please where the diodes must be used? (Sorry for these questions - I'm an electronics newbie).

You're right, it's probably better to use 1.2 volt batteries (as shown on your website). I'm going to build me an additional housing for these batteries.
Thanks again for the advice!
Regards
Dirk

[DIVER]

Flat coil wire gauge

The coil type that gave overall best results is a flat spiral wound coil - ID 14cm OD 24cm - 30 turns: [ATTACH]22997[/ATTACH] But without the shielding tape ! This is the coil that can be seen on one of the detectors on the picture at the top of this topic, embedded in a flat plate. So far, I only tried this coil and a 20 cm classic round coil - the other one on the picture. It's also surprising (or not) that the small euro coins, that are made of iron, can be as easily detected as the larger ones.

Hi Bernard,

Im waiting for parts to arrive. in the meantime may i know the wire gauge that you used for the flat coil? Thanks.

Vic