# Enhancing Meteor Observations with Photodiode Detectors
## 3 Concept
### Electronic Circuits  
The detector was built based on a small dark current (1 nA) photodiode of the PS100-6b type, shown as a D1 element in Figure 3. The detector circuit is made up of a transimpedance amplifier, an analog anti-aliasing filter based on an amplifier, and a digital signal processing circuit in an ADuC7020 microcontroller. The detector schematic is shown in Figure 3. The transimpedance amplifier was realized classically on a TLC272 operational amplifier marked on Figure 3 as U2, which converts the photodiode current to a voltage on a resistor of 20 M\(\Omega\). The resistor was chosen to ensure that the U2 amplifier is not saturated under typical nighttime conditions. The diode (detector) was polarized with a reverse voltage of 1.2 V, which provides a small junction capacitance and thus a higher response rate (a circuit with U1 diode produces this 1.2 V voltage). The signal from the transimpedance amplifier (U2) is given to the input of a low-pass, antialiasing Sallen-Key filter with a gain of 10 and Bessel-type characteristics. The Bessel characteristic was chosen to provide the step response of the filter without oscillations. Possible oscillations of the filter itself could deform the waveforms coming from objects in the sky. The full characteristics with step and impulse response of this filter are given in Figure 4. To maintain consistency across time-domain plots within the paper, and due to  
Figure 3: Detector diagram. Figure 2: The meteor detection system: four diode modules (**top**), ADuC circuit board (**bottom left**), IntelNUC computer (**bottom right**), and ASI ZWO178 camera with 2.5 mm fisheye lens (**right**). the discrete nature of the signal processing involved, we present both step and impulse responses as functions of sample number at a fixed sampling rate of 100 Hz, which is the effective rate of collected data. The output signal from the low-pass Sallen-Key filter is connected to the input of an A/D converter in the ADuC7020 microcontroller. The converter multiplexedly samples successive channels at a rate of 8 ksps and a resolution of 12 bits. Effectively, each channel is thus sampled at a rate of 2000 Hz with a resolution of 1.22 mV. The sampled signal is filtered with digital filter of IIR type (infinite impulse response). The role of this filter (in contrast to Sallen-Key which is antialasing filter) is the reduction of noise associated with the A/D conversions and increasing measurement resolution (smoothing the final signal). The first-order IIR can be described by the following differential equation and the schematic of the IIR filter algorithm is given in Figure 5.  
\[y[n]=a\cdot x[n]+(1-a)\cdot y[n-1]. \tag{1}\]  
This filter acts as a discrete equivalent of a first-order RC low-pass filter. The parameter \(a\) is equivalent to the time constant of the filter--the smaller the value of \(a\), the longer the  
Figure 4: Characteristics of antialiasing Sallen–Key filter. Figure 5: Schematic of digital low-pass filtering in a first-order IIR filter and decimation to 100 Hz. time constant. The choice of the parameter \(a\) involved some compromise in the system. Smaller values of \(a\) resulted in lower noise of the recorded waveform at the price of a slower step response, while larger values of \(a\) gave a faster step response with higher noise. We selected a value of \(1/16\), which does not disturb the signals (its cutoff 3 dB frequency is about 20 Hz while it is 7 Hz for analog antialiasing filter) while reduce mainly gaussian and 50 Hz noise before sending data to PC. The samples from these filters are decimated by 20 to have a final rate of 100 Hz. The value of 100 Hz allowed us to send all the required information from the module at the maximal speed of RS232 port--115.2 kbps. It appeared to be also satisfactory from the point of view of the observed phenomena and their dynamics. An additional advantage of this filter is that the output data samples are float numbers instead of 12-bit numbers (with 1.22 mV resolution), as received from the A/D unit.