TM 808 White's

[berreizeta]

[berreizeta]

RX SIGNAL 14V :


PROBE COILS (6.533Hz) :

[berreizeta]

Pito, join both coils at side, not at center. :

[berreizeta]

Tested with Active XTAL 25.6Khz (6.400Hz) and Pasive Xtal 420Khz (6.535Hz) both works fine !

25.6Khz + CD4024 = 6400Hz
420Khz + CD4040 = 6535Hz

Little change on circuit for 420Khz (photo), original circuit s/schematic don't work for me ..

[pito]

On collector Q1 20V (on the bottom ), on Rx testing coil 0.2 V = 5 time less than in DD coils = not good.

[pito]

or 26.3 kHz,
I don't think the exact frequency is needed, the device can work between 5kHz and 8kHz

[pito]

To generate 6.5 kHz you can use attiny85 and this program

#include <avr/io.h>

#include <avr/interrupt.h>

 

void comp_match() {

  DDRB |= (1 << PB0);  // set PB0 as output

  TCCR0A = 0x00;

  TCCR0B = 0x00;

  TCCR0B |= (1 << CS00);   // No prescaling

  TCCR0A |= (1 << WGM01);  // Toggle mode and compare match  mode

  OCR0A = 75;              // Compare value can be calculated as frequency = Clock / OCR0A

                           // A value of 75 leads to about 108,6 kHz at a clock rate of 8150000 Hz

  TCNT0 = 0;

  sei();  // Enabling global interrupt

  TIMSK |= (1 << OCIE0A);

}

 

ISR(TIMER0_COMPA_vect) {

  PORTB ^= (1 << PB0);  //Toggling port PB0

}

 

void setup() {

  comp_match();

};

 

void loop() {

}

 

/*

    

    The expected frequency can be calculated as freq = clock rate  / compare value

    In this specific case the clock rate is around 8 150 000 Hz.

    (See frequency 81 kHz for compare value 100)

    Empirical data measured 2023-08-23 with a noname ATtiny 85 board

    compare value    Frequency

            1..37 -> 217.3 kHz  

               38 -> 211,5kHz  

               40 -> 201 kHz

               50 -> 162 kHz

               60 -> 135 kHz

               70 -> 116,3 kHz  

               75 -> 108,6 kHz  

               76 -> 107,3 kHz  

               80 -> 102 kHz  

              100 -> 81,8 kHz  

              200 -> 41,1 kHz

              255 -> 32,3 kHz

*/
​

[ata]

Hi berreizeta
Have you finished making 808? And please announce the results. Thank you

[berreizeta]

Some modifications:

Power Supply changed 78S40 -> MC34063



Audio Amp :



Audio Amp PCB :



Batt Meter Resistor :

[berreizeta]

Coils dimensions are correct ?

[berreizeta]

I have changed the op amp from LM386 to a NE5532 and I think it works better.

Circuit :



Signal at CD4024 :



Signal Q1 to coil (6400Hz) :

[Skippy]

A curious observation. Normally, I would say 15V ( +/- 7.5V ) was chosen because that is near the maximum supply voltage for CD4000 series parts, and they perform best with higher voltages - faster switching, more drive current from the output, lower ON-resistance of analogue switches ( 4053, 4066 etc ) etc. And analogue circuits are able to handle higher level signals. Plus, back when this circuit was designed, a 15V analogue supply was quite normal; op-amps for 5 V or 3.3V operation were rare, so no-one would intentionally choose a low supply voltage, unless low power consumption was a design requirement.
The purpose of transistor Q1 is as a current source. The 4.3 V zener diode means the base voltage is fixed when it's turned on, and so the emitter is also reasonably fixed, 0.65V higher. So the 51 Ohm resistor has a steady ( 4.3 - 0.65 ) = 3.65V across it, and so passes a current of 72 mA. This should be the same for 12V or 15V supply.
One difference is the base current drive, through the 2K7 resistor. At 15V supply, there should be (15 - 4.3 ) / 2700 = 4 mA. At 12V, this drops to 2.9 mA. This may be important.
One experiment to try : test the circuit at 15V, with the 2k7 resistor. Then test it at 12V, but with a 2k0 resistor. These are both giving 4 mA base current. You could also try 15V and a 3k6 resistor, which gives 2.9 mA base current, and compare to 12V / 2k7 .
I believe the sinewave voltage on the coil is 'clamped' by the conduction of the transistor collector-base diode. The clamping is affected by the 2k7 resistor, and the zener diode, which forward conducts with 0.65V drop to the +ve supply rail.

On a different subject : I see your toroid is a bright green colour. Is this one you bought, to a particular specification, or is it a salvaged part ? It looks like a typical "common mode choke" , found on mains power supply filters in many circuits. If this is so, it may not be the best choice of ferrite material, as it is designed to absorb energy at 'high' frequencies. High may be just a few kHz, they are designed to pass 50 / 60 Hz, anything else can be filtered . They are probably not the best choice for this detector.

[berreizeta]

Done !
In the collector of Q1 I have 6.4Khz
With 15-16v the signal loses a couple of volts, maximum signal at 10-12v.
Why are 15 volts needed and it is not powered at 12v?
Tested with a small coil tuned close to 6400Hz *6398.4* and the Visual Analyzer program.

[Skippy]

Yes, see my post number 152. Your module generates a digital signal, the '4024 divider chip needs a digital signal. So you need a digital circuit, usually known as a "level shifter" to interface 3V3 ( or 5V ) signals to 15V signals. Luckily, the frequency of 26 kHz is pretty low, so that makes your level-shifter design simpler; fast switching circuits are usually more complex.
To save me the bother of drawing circuits and uploading them ( I have major problems with this new VBulletin 5.7 ), I will link to this article:

https://next-hack.com/index.php/2020/02/15/how-to-interface-a-3-3v-output-to-a-5v-input/

Figures 10, 11, 12 are the sort of schemes to use, just replace the +5V supply with the +7.5V.
There are other alternatives. If you use the virtual 0 Volt supply, you could power the osc module from a 5V regulator fed from +7.5V and 0V. Then a high-side pnp transistor based switch will switch between +5V and -7.5V, which is good enough to feed into a 4024 powered from +7.5V and -7.5V.
... that one needs a drawing, I guess.
That's why I said building your own 3.2768 MHz oscillator / divider, powered from 15V ( +7.5 and -7.5 ) would be a neater way of doing it.

[berreizeta]

The tests I've done with the 25.6Khz xtal lead me to add: a 78L33 (3.3v) to power the crystal

and a LM386 to raise the crystal signal to +-10V,

otherwise the 4024 at 16v won't work with xtal signal = 3.3 or 5V.