Electronic Design

IR sensor wakes up larger system

The sensor/monitor circuit shown draws so little supply current that it can remain on continuously in a notebook computer or PDA device (see the figure). Its purpose is to “wake up” the host system when infrared (IR) signals are detected. The circuit’s ultra-low current drain (4 µA maximum, 2.5 µA typical) is primarily that of the comparator/reference device IC1.

The circuit is intended for the noncarrier systems common in Infrared Data Association (IrDA) applications, and it operates with carrier protocols such as those of TV remote controllers and Newton/Sharp ASK (an Amplitude Shift Keying protocol developed by Sharp and used in the Apple Newton). The range for 115,000-baud IrDA is limited to about six inches, but for 2400-baud IrDA, it improves to more than one foot.

Bright flashes usually cause false triggers. Otherwise, the immunity to ambient light is very good. To handle occasional false triggers, the system simply looks for IR activity after waking and then goes back to sleep if none is present.

The sensor shown (D1), a relatively large-area photodiode packaged in an IR-filter material, produces about 60 µA when exposed to heavy illumination (and 0.4 V when open-circuited). Most such photodiodes are acceptable in this circuit. Operation is in the photovoltaic mode (without applied bias). This mode is slow and not generally used in photodiode circuits, but speed isn’t essential here. The photovoltaic mode simplifies the circuit and saves a tremendous amount of power. In a more conventional (photoconductive) configuration, photo currents caused by ambient light and sourced by the bias network would increase the quiescent current about ten times.

VREF and the R3/R4 divider introduce an 18-mV bias between the comparator inputs. Derived from the reference, this bias is independent of the supply voltage. To suppress 60-Hz/120-Hz hum and other low-frequency disturbances, C3 and the R3/R4 divider form a high-pass network whose cutoff frequency is 700 Hz. C3 is normally charged to VREF minus the 18-mV bias, and any voltage produced by photocurrent through R2 adds to the voltage on C3.

Consequently, any IR signal across R2 that exceeds the 18-mV threshold trips the comparator and causes its output to go low (18 mV represents a good trade-off between range, noise immunity, and dc stability). The low value of R2 prevents saturation of the photodiode in ambient light. If saturation is an issue, the R2 value can be lowered further¾with a penalty in sensitivity and a boost in speed.

The comparator’s input offset voltage (10 mV maximum) sets worstcase extremes of 6 mV and 28 mV for the IR trip threshold, but this spread isn’t as big an issue as one might imagine. Typical spreads are much smaller than the maximums, and typical IR signals generate more than 60 mV. A variation in offset affects the amount of overdrive, which therefore affects only the comparator’s speed of response.

The circuit’s output can trip a setreset flip-flop or interrupt a sleeping processor. The optional HCMOS gate (preferably a Schmitt-trigger type) can improve the output rise/fall times with very little effect on the overall quiescent current.

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