Modification of Harbor Freight Centec handheld metal detector Cen-tech #97245

Some quick mods of the ‘pinpointer’:

·         Add radioshack vibrate motor

·         Add switch selector for original piezo buzzer or vibrator

·         LED straightened and brought out top of case

o   LED now lights when metal detected and not a ‘flashlight’ anymore.

o   Sink the LED with a 1k resistor versus the oem 300 ohms

o   Current draw with no detect is < 5ma now (no LED lit)

o   With piezo selected (and LED lit), current draw is ~45ma

o   With vibrator motor (and LED lit) current draw is about 38ma

·         Add external adjustment for the internal balance potentiometer

·         Add larger knobs for the sensitivity adjustment and external adjustment

 

Click here for effect of temperature on the pinpointer

Given the schematic diagram from here (A super BIG thank you to whoever provided the original!!!):

Original Schematic

With the mods and additions shown here:

 

The case outside, notice the LED now shines through just above the ‘Light’ label.

The added knob on the oem sensitivity and the new external adjust.

The slide switch for piezo or vibrator select on the right.

Replaced the oem pushbutton switch on the left.

Side view showing the added vibrate motor/piezo selector switch. I have a micromill so cutting the slot was easy.

 There is enough lower part of the case above the PCB to fit the switch

 

Up-close on the switch, can also see the 1k resistor for the led from the schematic above.

The cut for the jumper shown on the schematic is a solder blob as shipped between the two feedthroughs just below ‘BZ’ seen below.

The buzzer add, also showing R14 for the 220 ohm resistor (just left of the coiled red wire)

There is already a place on the PCB to mount the buzzer motor. I coiled the wire because I never cut if not required J

 

Showing left LED leg cut and LED straightened to bring out top of case. Wire runs over to the 1k resistor in the switch section above.

 

Adding external control for the internal potentiometer.

This device is extremely affected by temperature.

The NTC thermistor shown in the schematic, but absent from the device, is really needed, or else something to compensate for temperature changes is required. The internal potentiometer provides this capability if the pot is brought out external to the case so you can tweak as required to obtain the max distance, or at least the same distance across temperature.

The nub end of the external adjustment knob, adding the black rectangle ‘washer’ allows a force fit of nub end into the potentiometer adjustment slot when the case is screwed together

 

The internal potentiometer, with a captive adapter to hold the nub end in the slot so cannot shift side to side.

 

Measuring temperature dependency

Using a handheld non-contact thermometer aimed at the case (much better would be at the PCB!!)

Setting sensitivity and new external adjust to ‘max’ at room temperature (69F)

 

With the initial adjustment of external to ‘max’ at room temperature, temperatures above 70F just ‘detect’ continuously,

the external adjust must be ‘adjusted’ for new higher temperature if you want max range.

 

Copyright (all but schematic) astrogene1000@2012
Copyright schematic original author