If you face the challenge of adding a second, independently controlled light source to an existing ceiling lamp controlled by a wall switch, you may find that stringing a second power line is impossible. First, you can replace the wall switch by the circuit in Figure 1. Pushing the on switch S₁ or S₂ for approximately 1 sec inserts the 12V zener diodes D₁ or D₂ in series with the hot wire of the power line. During the push, the polarity-dependent conduction of the zener diodes creates a small positive (negative for D₂) dc component across the line and only slightly reduces the line's 120V-ac component. A control circuit at the lamps' site reacts selectively to the polarity of this dc pulse and controls the power to the two lamps. The required power rating of the two zener diodes depends on the load current. The short duration and low duty cycle of the activation are helpful. The 1N2976 diodes in Figure 1 are rated for continuous dissipation of 10W.

Figure 2 shows the first part of the control circuit located at the lamps' site, including the two leads of the power line, W₁ and W₂. Current through capacitor C₁ and resistor R₁ creates a 60-Hz square wave across the 6V zener diode, D₃. Diode D₁ and filter capacitor C₂ generate a dc supply voltage of V_DD=5V for the control circuit's active elements. Two two-stage RC filters connected to W₂ create V₁ and V₂, with reference to W₁. The filters attenuate the 120V-ac voltage between W₁ and W₂ to a subvolt level in V₁ and V₂. An extra zener diode, D₄, creates a positive 5V dc bias in V₂. The filter outputs V₁ and V₂ drive inverting Schmitt triggers IC₁ and IC₂. Inserting zener diode D₁ by pushing S₁ in Figure 1 changes V₁ from 0V to 5V and V₂ from 5V to 10V. Inserting D₂ in Figure 1 by pushing S₂ changes V₁ from 0V to –5V and V₂ from 5V to 0V. Note that the input-protection diodes of IC₁ and IC₂ limit the voltage swings of V₁ and V₂. The output V₃ of IC₃ responds to pushing S₁ by a positive transition and has no response to pushing S₂. The output V₄ of IC₂ responds to pushing S₂ by a positive transition and has no response to pushing S₁.

Figure 3 shows the second part of the control circuit located at the lamps' site. Signals V₃ and V₄ in Figure 2 drive the clock input of toggle flip-flops IC₁ and IC₂, respectively. For clarity, Figure 3 doesn't show the connections of the flip-flops of  to D and the Set terminal to V_DD. When you push switch S₁ in Figure 1, the positive transition in V₃ toggles flip-flop IC₁. Similarly, when you push S₂, the positive transition in V₄ toggles flip-flop IC₂. Thus, you can independently control the states of flip-flops IC₁ and IC₂ by pushing S₁ and S₂, respectively. To drive the two lamps, the Q outputs of the flip-flops drive the gates of triacs TR₁ and TR₂ via coupling resistors R₂ and R₃. The MT2 terminal of each triac drives the lamps, L₁ and L₂, respectively. Pushing S₁ changes the state of lamp L₁; pushing S₂ changes the state of lamp L₂. Thus, you have independent control of both lamps on a single power line. In this application, you want to keep each lamp's terminals safely connected to the hot and neutral wires. Therefore, you make W₁ the hot wire and W₂ the neutral wire.

With the control circuit, the state of the flip-flops becomes uncertain if, after an interruption, the ac power returns. This situation is unacceptable because the lamps could turn on and stay on for an uncontrollable length of time. Therefore, you add a power-up reset circuit (Figure 3). To guarantee a safe reset also for short interruptions, the reset circuit must quickly pull down the flip-flops' Reset terminal, which is independent of V_DD's slowly dropping level. Diode D₂ (driven by the 60-Hz square wave across zener diode D₄), capacitor C₃, and resistor R₄ act as an auxiliary rectifier supplying voltage V₅. When power experiences an interruption, V₅ drops to 0V much faster than V_DD. V₅ drives the cascade of inverting Schmitt triggers IC₄ and IC₅, which then quickly pull down the flip-flops' Reset terminals via diode D₃. When power returns, the Reset terminals pull up slowly via resistor R₅. The R₅ C₄ time constant guarantees that the flip-flops' Reset does not release before V_DD reaches its full value.

Note that you can use this Design Idea in other applications. For example, if you omit the circuit in Figure 3, the transitions in V₃ and V₄ can drive an up/down counter that can perform an auxiliary control function for a device that the ac line powers. If you insert an additional conventional switch in series with the circuit in Figure 1, it can turn on and off the power to the device. If the application requires control signals near ground level, wire W₁ should be the neutral lead of the power line, and W₂ should be the hot lead. However, make sure that the powered device is not an inductive load because it can short out the controlling dc pulses.

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