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Hello boys!
It's been a while since I contributed a little thing to the forum and I apologize for that.
After installing the components, the circuit wasn't working at all except for the oscillator part and I got a frequency of about 96.6 kHz at the TX point. So I had to dig deeper into it. This is going to be a two parter:
Part 1:
I found out that the voltage at pin#9 of IC1, where it should be Ideally 3V (half of the supply voltage 6 V), was near zero. Which means the IC (CD4046) was "out of lock". Therefore, I had to replace R7 with a 100k pot to raise the minimum frequency for the VCO. After turning the pot to almost mid way -around 47k- the circuit came to life. However, it was only detecting big objects. In fact, the background sound faded away when I got a tiny metal object near the sensor. And that's what the second part is going to be about.
Part 2:
The reason that the coil wasn't sensing tiny metals is because the voltage at the base (0.7V) was lower than the emitter (1.2V) of Q1. So I knew I had to mess with R1, R2 (Q1 base voltage) and R3 (the emitter voltage) in order for me to get the voltage of the base to be higher than the emitter voltage by exactly 0.7 V which is the bias point (Vbe) of the transistor I'm using for Q1 (2N2222). The changes to the mentioned resistors were as follows: R1= 10k, R2= 10k pot, and R3= 2k pot. So I managed to make the voltage at the base 1.9 V and at the emitter 1.2 V and now you bet the circuit was fully functional!
Unfortunately I didn't desolder the pots so I can not give you the exact values for the resistors But at least now you know where to look (R1, R2, R3, R7)
I hope either one of the authors of ITMD3 can verify this.
Attached is the worst lay file design for the circuit!
Cheers,
It's been a while since I contributed a little thing to the forum and I apologize for that.
After installing the components, the circuit wasn't working at all except for the oscillator part and I got a frequency of about 96.6 kHz at the TX point. So I had to dig deeper into it. This is going to be a two parter:
Part 1:
I found out that the voltage at pin#9 of IC1, where it should be Ideally 3V (half of the supply voltage 6 V), was near zero. Which means the IC (CD4046) was "out of lock". Therefore, I had to replace R7 with a 100k pot to raise the minimum frequency for the VCO. After turning the pot to almost mid way -around 47k- the circuit came to life. However, it was only detecting big objects. In fact, the background sound faded away when I got a tiny metal object near the sensor. And that's what the second part is going to be about.
Part 2:
The reason that the coil wasn't sensing tiny metals is because the voltage at the base (0.7V) was lower than the emitter (1.2V) of Q1. So I knew I had to mess with R1, R2 (Q1 base voltage) and R3 (the emitter voltage) in order for me to get the voltage of the base to be higher than the emitter voltage by exactly 0.7 V which is the bias point (Vbe) of the transistor I'm using for Q1 (2N2222). The changes to the mentioned resistors were as follows: R1= 10k, R2= 10k pot, and R3= 2k pot. So I managed to make the voltage at the base 1.9 V and at the emitter 1.2 V and now you bet the circuit was fully functional!
Unfortunately I didn't desolder the pots so I can not give you the exact values for the resistors But at least now you know where to look (R1, R2, R3, R7)
I hope either one of the authors of ITMD3 can verify this.
Attached is the worst lay file design for the circuit!
Cheers,
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