How to Use Op Amps to Build RC Timing Circuits

When slight pressure is applied to a fiber to make it into a V-shape, a battery-powered, handheld "fiber finder" measures the light escaping from it. A pair of photocells compares analog levels on either side of the bend to indicate the presence and direction of light transmission, and a PLL tone decoder indicates up to three optical modulation tones. The idea is to "tag" a fiber with a signal from a switching center so that an operator on a utility pole or in a manhole can find and correctly identify the fiber before making cuts and splices, thus avoiding unexpected failures.

Because there is no room on the front panel for a power switch, the design requires a sliding clamping mechanism that powers up when the operator inserts a fiber and stops when it is fully inserted. The mechanism must remain on each time the operator inserts another fiber, and it automatically turns off when the operator is done and no longer activates the clamping slide. This design does not have room for a bulky multi-pole switch; it is suitable for single-pole operation. The design uses phosphor bronze wires that rest on a gold post on the PCB, making it a nearly cost-free, no-body switch. This function does not use a processor or digital clock, but uses a spare op amp and a few components (Figure 1).

Figure 1. This circuit uses an RC timer to turn on power for a predetermined length of time each time the momentary switch contacts close.

S1 is a normally open contact. When unpowered, any residual charge on C1 is dissipated through R5 and D1, a low leakage switching diode such as the MMBD2836, which is housed in a common anode package with D2 to block input current from passing through IC1 to the power rail. PNP transistor Q1, which pulls current out of the device, remains in the off state; the voltage drop across R1 due to the current in R4 is too low to bias Q1 on. Since the output of unpowered IC1 is zero, Q2 is off.

Closing S1 biases Q1 into the on state, thus powering up the regulator and providing power to the rest of the device. Closing S1 also ensures that C1 is fully discharged through R5 and D2. Now, IC1 is operating normally, and its positive input is biased at 60% of the battery voltage through R6 and R7, which is the voltage after about one RC clock constant. IC1 is a single-supply CMOS device, such as the LMC6482, a rail-to-rail op amp with low-leakage inputs. A CMOS comparator with low-leakage inputs can also be used; if it is an open-collector output, an R10 pull-up resistor must be added.

While C1 remains discharged, IC1's output approaches the upper supply rail, turning on Q2, which can be any low-leakage, general-purpose NPN transistor, such as the MMBT3904, or an N-channel enhancement logic-level FET. After S1 opens, Q2 maintains current through Q1's base, keeping it energized.

When S1 opens, C1 begins to charge to the base voltage of Q1 through R3, R4, and R5, and the base-emitter voltage drops below the battery. S1 then closes, discharging C1 and restarting the timer. When S1 is open for longer than the RC time constant of C1R4 (approximately 10 seconds in the figure, with the values ​​of R3 and R5 negligible), the voltage at the input of IC1 rises above the positive input, and the output of IC1 drops to nearly ground. This action turns off Q2, which in turn turns off Q1, and the device is de-energized. As the rail voltage drops, C1 discharges through D1 and R5 to prevent the clamping diode from damaging the negative input of IC1, but remains close to the rail. The positive input of IC1 is always equal to 60% of the rail, ensuring that the output of IC1 is always below the rail. Adjusting R8 and R9 limits the base voltage of Q2 to below its turn-on threshold to prevent any output glitches that may exist from the op amp or comparator.