Can't identify electronics symbol

Thread Starter

DonBradman

Joined Mar 30, 2023
40
In the schematic below, I am unable to understand what is that symbol with the arrow across a circle adjacent to a M. Also, in the op amp, possibly LM118, where is the 27K going, in which pin?

1680180581448.png
 

Irving

Joined Jan 30, 2016
3,843
That looks like a good old fashioned D’arsonval meter. The line doesn’t usually cross the circle, it remains inside, but otherwise that’s what I would expect.
Probably so, but maybe for practical purposes its a voltmeter (25uA FullScale microammeter in series with 1Meg preset - at 500k -for calibration), measuring the voltage across RV1 (5k) and the 1k. This suggests a LDR at 2.5k approx for illumination of 20 - 30Lux. This gives a current of approx 5mA through the LDR and a reading of 10 - 15v (nominally 12v) on the meter. Replace the meter and the 1Meg preset with a digital voltmeter with a >500kohm input impedance. Its likely the original meter had a logarithmic scale calibrated in Lux.
 

Thread Starter

DonBradman

Joined Mar 30, 2023
40
Hi Don,
This is the full Compensation circuit.
Where did you get the circuit diagram?
E
EView attachment 291077.?
This schematic was from an old research paper I found on pulse identification system. They do not provide any elaboration over it, either in text or schematic. Interestingly their schematic, as you can see, has only one 27K, while the full compensation circuit has two 27Ks. Why could that be so?
 

Thread Starter

DonBradman

Joined Mar 30, 2023
40
So what is the purpose of this circuit? Its an odd one...
Its a heart rate identification circuit for animals. Could you help guess where the 27K is going? Pin 1 or Pin 5? Or would it be safe to attach a 27K to both pin 1 and pin 5 even though the schematic doesn't say so
 

Irving

Joined Jan 30, 2016
3,843
Having too much time on my hands today (otherwise known as procrastinating about the next thing I'm supposed to be doing) I was intrigued by the circuit, which turns out to be a hi-pass filter (green trace below) followed by a low-pass filter (red trace, and obvious by inspection of the placements of Rs and Cs in the feedback loops). I've added R10 at the base of T6 otherwise it doesn't really work properly - I think the schematic got creased at T6 and this was missing in action. The multiple traces represent the adjustment of X1 and X2, the 10k pots..

1680264290225.png
 

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Thread Starter

DonBradman

Joined Mar 30, 2023
40
Having too much time on my hands today (otherwise known as procrastinating about the next thing I'm supposed to be doing) I was intrigued by the circuit, which turns out to be a hi-pass filter (green trace below) followed by a low-pass filter (red trace, and obvious by inspection of the placements of Rs and Cs in the feedback loops). I've added R10 at the base of T6 otherwise it doesn't really work properly - I think the schematic got creased at T6 and this was missing in action. The multiple traces represent the adjustment of X1 and X2, the 10k pots..

View attachment 291157
wow, firstly, that image on the right is super cool. Secondly, I need help identifying the part where the HPF starts and end and where the LPF starts and end so I can modify it. I usually use opamps for the filters, not transistors, so this is confusing me. In my limited understanding of electronics, any filter has a resistor and a capacitor, the cutoff set by the formula 1/2PiRC, but this circuit is supposed to have a HPF of 8.3Hz and a LPF of 13.3Hz, and I can't find the Capacitor Resistor combinations in the schematic that make that math.
 

Tonyr1084

Joined Sep 24, 2015
7,852
in the op amp, possibly LM118, where is the 27K going, in which pin?
Would appear to be going to pin 7. That is IF I chose the correct manufacturer's data sheet. Not all manufacturers follow the same practice. Nevertheless, the 27K is going to the positive power pin (V+) for that IC.
{edit} No, it's not the power pin. Sorry for the misleading information. Apparently it must be going to one of these three pins: Pin 1, Pin 5, Pin 8. So I don't know which pin it's going to. I'm sure someone else can figure it out.
1680275677950.png
 
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Irving

Joined Jan 30, 2016
3,843
T1-T3 are the hi-pass, T4-T6 are the low-pass. The point where they join is marked hi-pass on my schematic above. Below, I've split them out and swapped round X2 so it has the same' direction' as X1

Personally I'd junk them and use a 2nd- or 3rd-order opamp filter for each...

1680273865461.png
 

Thread Starter

DonBradman

Joined Mar 30, 2023
40
T
T1-T3 are the hi-pass, T4-T6 are the low-pass. The point where they join is marked hi-pass on my schematic above. Below, I've split them out and swapped round X2 so it has the same' direction' as X1

Personally I'd junk them and use a 2nd- or 3rd-order opamp filter for each...

View attachment 291172
Thanks, I think I'll stick to the opamps too. One thing which has been bugging me, I tried to modify another schematic, Ill post the picture below. When I went exactly by the schematic the output was fine and the reaction was instantaneous, but I tried to reduce the HPF to 0.5Hz and increase the LPF to 50Hz with very small capacitors (10nF Ceramic on each) and very large resistors, I noticed that the reaction time became slow to the pulse in my finger (Also for some reason even with no finger the analog output would periodically suddenly rise and slowly fall and repeat). If I give the sensor direct light, like if I remove its cover and expose it to room light, the jump in the analog reading is instantaneous, but if I put my finger it takes 5-6 seconds for the pulse to appear, which is terribly underperforming outcome. Why could that be? Is it that the amplification at the LPF stage is too high for it to react quickly? Should I stick with smaller resistor at LPF(but then the capacitor would become larger and I fear it might take longer to charge and my system would still react slow). Where am I exactly going wrong? Original schematic below, my modifications mentioned in text above.
1680281596505.png
 
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Thread Starter

DonBradman

Joined Mar 30, 2023
40
I found the potentiometer for LT Spice here

But whenever I try to paste the asy file into my sym folder, I get the error
The Compressed zipped folder is invalid or Corrupted
 
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