Greetings,
Maybe I can finish up with this installment. Since I reduced the gain in the receiver preamp It is obvious that I need to get the gain back up in the back part of the circuit. After the target signal and the second sample are gated through the gating IC they are amplified 330 times but the 1st differential integrator. To obtain the best common mode rejection there are 4 resistors that should have a 1% tolerance. They are marked with and asterisk. I actually used 5% resistor and they have worked fine for me but 1% would be the best choice. The TL072 is a dual Cmos Op-amp and the integrated output of the 1st differential integrator is capacitive coupled into the + input on the second half of this IC. The Cmos TL072 IC and capacitive coupling it allow for the ability to make a variable SAT (Self Adjusting Threshold). A great idea in Carl Morland's Hammerhead. This along with the lower preamp gain is what allow the detector to work well even when in reasonably bad ground. The - input gets a voltage input from the Threshold control. I also added a Gain control that allow the gain to be varied about 15 to 1. The total gain from the preamp to the output of the second integrator is 15 million at max gain and 1 million at the lowest gain. The integrated output of the 2nd Integrator feeds the audio through a Low Pass filter.This filter helps get rid of higher frequency noise but still let target signals pass through. I measured my normal sweep speed and calculated the time that it would take the coil to pass over a small target, then set the roll off of this filter to insure that I would not filter out the target. I kind of tested my design by insuring that the detector would double beep when I passed over a bobby pin length wise. In my mind the best value for the Low Pass capacitor is between 1 uf and 5 uf. I am using 5 uf and have had no trouble hearing small gold targets. The two diodes that are in series and in parallel with the 5 uf cap is to clamp the output of the last integrator and limit the highest frequency that the VCO audio will go to on large or shallow targets. This give the detector a very good audio response and generally a good sound. The output of the Low Pass filter drive a transistor current source into the timing circuit of a Cmos 555 timer to generate the VCO audio. This stage is pretty standard for VCO detectors. The second transistor in the circuit is hooked up as a diode and keeps the emitter of the current source at virtual ground and also provides temperature compensation.
The optional battery alarm is just an Op-amp hooked up as a comparator. When the battery voltage gets to a particular level the comparator switches and turns on the output transistor that dumps current into the middle of the filter driving it into the diode clamps and turning the audio full on until the power switch is turned off. Once the batteries are changed the detector will then function as normal. When the Set Pot is at one end it will switch the alarm on around 8 volts which is good for Nimh and alkaline batteries. when it is set to the other end it will switch about 11 volts and that is about right for a 4 cell 14.8V Lipo battery.
That is it, feel free to ask any questions and feel free to comment about any of it.
God Bless and HH.
Cyril
Maybe I can finish up with this installment. Since I reduced the gain in the receiver preamp It is obvious that I need to get the gain back up in the back part of the circuit. After the target signal and the second sample are gated through the gating IC they are amplified 330 times but the 1st differential integrator. To obtain the best common mode rejection there are 4 resistors that should have a 1% tolerance. They are marked with and asterisk. I actually used 5% resistor and they have worked fine for me but 1% would be the best choice. The TL072 is a dual Cmos Op-amp and the integrated output of the 1st differential integrator is capacitive coupled into the + input on the second half of this IC. The Cmos TL072 IC and capacitive coupling it allow for the ability to make a variable SAT (Self Adjusting Threshold). A great idea in Carl Morland's Hammerhead. This along with the lower preamp gain is what allow the detector to work well even when in reasonably bad ground. The - input gets a voltage input from the Threshold control. I also added a Gain control that allow the gain to be varied about 15 to 1. The total gain from the preamp to the output of the second integrator is 15 million at max gain and 1 million at the lowest gain. The integrated output of the 2nd Integrator feeds the audio through a Low Pass filter.This filter helps get rid of higher frequency noise but still let target signals pass through. I measured my normal sweep speed and calculated the time that it would take the coil to pass over a small target, then set the roll off of this filter to insure that I would not filter out the target. I kind of tested my design by insuring that the detector would double beep when I passed over a bobby pin length wise. In my mind the best value for the Low Pass capacitor is between 1 uf and 5 uf. I am using 5 uf and have had no trouble hearing small gold targets. The two diodes that are in series and in parallel with the 5 uf cap is to clamp the output of the last integrator and limit the highest frequency that the VCO audio will go to on large or shallow targets. This give the detector a very good audio response and generally a good sound. The output of the Low Pass filter drive a transistor current source into the timing circuit of a Cmos 555 timer to generate the VCO audio. This stage is pretty standard for VCO detectors. The second transistor in the circuit is hooked up as a diode and keeps the emitter of the current source at virtual ground and also provides temperature compensation.
The optional battery alarm is just an Op-amp hooked up as a comparator. When the battery voltage gets to a particular level the comparator switches and turns on the output transistor that dumps current into the middle of the filter driving it into the diode clamps and turning the audio full on until the power switch is turned off. Once the batteries are changed the detector will then function as normal. When the Set Pot is at one end it will switch the alarm on around 8 volts which is good for Nimh and alkaline batteries. when it is set to the other end it will switch about 11 volts and that is about right for a 4 cell 14.8V Lipo battery.
That is it, feel free to ask any questions and feel free to comment about any of it.
God Bless and HH.
Cyril
