Step 8

[Rinkerdink]

All working great at this point except I'm getting a voltage swing of -3.54 to 4.36 across the range of the 200 ohm pot that came with the kit. I'm thinking this is ok although it isn't the 5 to -5 vDC as described in the instructions?

any thoughts please ?

thanks

chris

[homefire]

I think that is within exceptable range. Try a Iron Target. Did you follow this ? " Also, either the Reject pot should be connected to PL3, or the pins shorted (as described in
Step 7)."


STEP 8
Second integrator (see Fig. 12):
Fit R30/R32/R33 (1k), R31 (27k), R29 (1M), R34 (47k), C15 (470nF), C18 (1uF), TP9, PL6, U9
(TL061), Audio [threshold] pot (250k LIN)
Monitor TP9 with an oscilloscope, and adjust the Audio (threshold) pot. This should allow the DC
voltage level at TP9 to swing from -5V to +5V. Set the DC level to around 0V. While still
monitoring TP9, look for a change in the DC voltage level as a target is moved across the coil. Note
the self-adjusting threshold (SAT) will restore the signal to its previous value if the target is not kept
moving. Also, either the Reject pot should be connected to PL3, or the pins shorted (as described in
Step 7).

HF

[Rinkerdink]

All that works although shorting the pins on pl3 doesn't seem to make any difference? I just added the reject pot and turned to about 5 ohms, still no difference. I do get a lovely reaction when targets are passed by the coil and the voltage drops off when they're left stationary so guess all is ok...

[homefire]

Yes the SAT is adjusting the Threshold. Target must be moving. Turn the pot all the way down.

[Rinkerdink]

Thanks for the help, just turned it on and my terrible coil is picking up coins at about a foot! Can't believe it works

[homefire]

Congratulations Sir. How wide is your coil ?

[JOHNCGRAY]

Just to let you know that I also did not get the -5 to +5v. I got -3.50v to +4.33v ie very similar to yours.

[Rayjay]

When I did that step, I got -4.0 to +4.0 volts.

[homefire]

Even using all 1% resistors, there are variances between components. You did good getting the symmetrical 4V.