با سلام
این مدار پیشنهادی منه ، البته بصورت عملی آزمایشش نکردم . روهوا سوارش کنید اگه جواب مطلوب گرفتین میتونین از برد ماتریسی استفاده کنید
مشاهده فایل‌پیوست 180938
اگه عقربه تا آخر حرکت کرد ، منظور اگه خروجی دستگاهتون خیلی قوی بود از پتانسیومتر 10 کیلو در خروجی استفاده کنید
معمولا نمایشگرهای عقربه ای با ولتاژ 0.2 ولت عقربه تا آخر حرکت می کنه، اگه امکانش بود با پاور نمایشگر خود را تستش کنید
مشاهده فایل‌پیوست 180939
این هم یه نمونه که تو اینترنت بود و با یه سری فرمول توضیح داده بودن
مشاهده فایل‌پیوست 180940
موفق و پیروز باشید
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First the AC signal across the speaker is rectified by diode D1. D1 should be a germanium diode or a Shottky diode to minimize the voltage drop across it. Every millivolt lost here will reduce the sensitivity at low volume. But D1 has to stand twice the peak voltage of the speaker, so the reverse voltage of the diode has to be selected accordingly. For example, 80 W over 8 Ω have a voltage of √ 80 W · 8 Ω = 25.3 V[SUB]RMS[/SUB] and a peak voltage of √ 2 · 25.3 V = 35.8 V[SUB]peak[/SUB], so the diode will get 2 · 35.8 V = 71.6 V on its leads. Germanium diodes have a low voltage drop, about 0.25-0.3 V, but the majority of them have breakdown voltages between 30 and 40 V! Small signal Schottky diodes can have voltage drops in the 0.2-0.4 V range depending on the model and can easily deal with reverse voltages up to 100 V. Don't use high current Schottky diodes or silicon diodes because their voltage drop is in the 0.5-0.7 V range and is too high for this application.
Resistor R1 is a very good idea because it increases the impedance of the meter and limits the (non-linear) current drawn by this circuits so that it can be neglected compared to the main speaker current. Otherwise, connecting a rectifier in parallel with a speaker could introduce some distortion. The 10 kΩ of R1 are high enough compared to the 8 Ω of the speaker to make sure that the total harmonic distortion won't be affected. R1 will also limit the current in D1 if the maximum diode reverse voltage is exceeded, preventing damages in case of small peaks. Capacitor C1 integrates the rectified signal and provides only DC to the meter. The time constant is below 50 ms, low enough not to slow down the meter that has usually a time constant in the 300 ms range.
Now the tricky part: for low voltages, D2, D3 and D4 do not conduct. These are regular small signal silicon diodes with a forward drop of 0.6 V; here a high voltage drop is important. So for low voltages D2, D3, D4, R2 and R3 can be neglected and the current can flow directly in the meter. This can be observed in the graph below (the one on the left): for input voltages below 0.6 V, the current in the meter (blue trace) is linear and D2 (green trace) and D3-D4 (yellow trace) conduct no current. As soon as the voltage rises, D2 starts conducting and robs current to the meter making it less sensitive to higher voltages. On the same graph, above 0.6 V the current in D2 (green trace) starts rising and the current in the meter (blue trace) rises with a lower slope. For even higher voltages (about 10 V, graph on the right), D3 and D4 will start conducting as well robbing even more current (yellow trace). The effect is that the meter current slope (blue trace) is further reduced approximating a nice logarithmic function. To calculate this circuit (especially the values of R2 and R3), the common approximation of the 0.6 V threshold voltage is not enough. One has to use the diode characteristic exponential function, but equations get complicated and I wasn't able to solve it with just a pencil and a piece of paper. Fortunately, free SPICE software is available and the circuit can easily be simulated and a reasonable solution can be found by modifying the values of these to resistors until a suitable response is achieved.
