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Threshold Detector for Optical Transducers

IP.com Disclosure Number: IPCOM000087677D
Original Publication Date: 1977-Mar-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 28K

Publishing Venue

IBM

Related People

Gianos, PT: AUTHOR

Abstract

This is a threshold detector circuit which allows for decreased or increased sensitivity of different transducers, or of the same transducer as it ages, and provides improved ability to identify a weak signal in relation to DC and/or 60-cycle noise. These improvements are obtained by providing a circuit in which the threshold level for the transducer signal may be designed to an unusually low value. A low threshold value is possible because the threshold is made to track low frequency noise, thus eliminating the normal lower bound on the threshold value due to expected ambient light. The threshold value may be set to allow for minimum lifetime transducer signals without regard for expected low frequency noise levels.

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Threshold Detector for Optical Transducers

This is a threshold detector circuit which allows for decreased or increased sensitivity of different transducers, or of the same transducer as it ages, and provides improved ability to identify a weak signal in relation to DC and/or 60- cycle noise. These improvements are obtained by providing a circuit in which the threshold level for the transducer signal may be designed to an unusually low value. A low threshold value is possible because the threshold is made to track low frequency noise, thus eliminating the normal lower bound on the threshold value due to expected ambient light. The threshold value may be set to allow for minimum lifetime transducer signals without regard for expected low frequency noise levels.

Referring to the figure, T1 is a phototransistor that is receiving an incoming light signal and producing a current. The grouping of R1, R2, RF and A1 form a current-to-voltage amplifier. The voltage divider of R1 and R2 biases the noninverting input of the amplifier A1 to a constant voltage. The negative feedback provided by RF maintains the inverting input of A1 at the same voltage as the noninverting input of A1. Neglecting the input bias current of the amplifier A1, the current signal from T1 must flow through RF. The signal current flow through RF causes the output voltage of amplifier A1 to vary according to the relationship V(OUT) = V(REF) - RF(I), where V(REF) is the constant voltage on the input terminals of A1. The current-to-voltage gain then is determined by the value of RF.

Any illumination signal into the phototransistor will have three components (a DC component, a 60 Hz component, and a high frequency component), assuming that the desired signal source is transmitting a high frequency pulsed or oscillating signal. The DC and 60 Hz components will be due to ambient light conditions, e.g., sunlight and fluorescent lights. The A1 output signal will be an amplified copy of the light signal input to the phototransistor. The signal at the inverting input of amplifier A2 will be the output of A1.

The combination of D1, R3, R4 and CF is the filtering network that accomplishes the variable thresholding necessary for this circuit to accomplish its objectives. If there is a change in the DC component of the input signal and the amplified signal at the output of A1, the change will be filtered out by capacitor CF and the DC level at the noninverting input of A2 will remain one diode drop (the drop across D1) below the DC level of the inverting input to A2. This diode drop...