Analog Circuit Description

I have not done a complete reverse engineering of the Photon system, but enough to give a basic description of what is going on. Double-click on any picture to see a full size image.

The LEDs are run in series with one current limiting resistor on the pickup PCB. Therefore if one emitter fails, all of them go dark. Measuring the forward voltage using the diode/continuity setting of a simple DVM without power applied to the pickup system will reveal the failed part. It will have no voltage drop while the functioning ones run about 1.4 volts.

The emitter of the phototransistors and photodiodes is tied to ground while the collector connects to the pickup interface board where there is a sensitivity trim pot for each detector. This signal is then sent through the 15 pin D cabling to the main box.

Signal from Pickup String aligned in center of light path	Signal from Pickup System String aligned below light path

Each string signal is AC coupled to eliminate the DC signal from the photo detector and connected to one half of a CA3280 dual Operational Transconductance Amplifier or OTA. There are a total of three 3280s in the controller.

The following paragraph is excerpted from the Intersil spec. sheet for reference.

"The CA3280 has all the generic characteristics of an operational voltage amplifier except that the forward transfer characteristics is best described by transconductance rather than voltage gain, and the output is current, not voltage. The magnitude of the output current is equal to the product of transconductance and the input voltage. This type of operational transconductance amplifier was first introduced in 1969, and it has since gained wide acceptance as a gateable, gain controlled building block for instrumentation and audio applications, such as linearization of transducer outputs, standardization of widely changing signals for data processing, multiplexing, instrumentation amplifiers operating from the nanopower range to high current and high speed comparators."

The transconductance (i.e. gain) of the OTA is controlled by a current input on pins 1 and 8. The Photon Pickup Sensitivity knob on the front panel adjusts the signal gain at this point.

From there the signal is routed to a LM1458 dual op amp which amplifies the signal . Section A is the amplifier and section B appears to be an inverter. Next in line is a National MF10 Universal Monolithic Switched Capacitor Filter. This is a dual filter and one filter is used for each string so there are three devices in the unit. This is a tuned low pass filter and it doesn't appear that there is any active adjustment of the filter based on the note played.

The filtered signal, which is fairly sinusoidal at this point, is then fed to an LM324 quad OP AMP. The B section of the 324 amplifies the signal from the low pass filter by about 2.  The A section then further amplifies and clips the signal to create a square wave.

Signal After Low Pass Filter	Signal after squaring

Section D of the LM324 does heavy integration to extract the envelope of the signal.

Envelope Signal After Integration (0.5 seconds per graticule)

Once squared the signal is run to a series of gates and counters.

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