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■ FIGURE 4 current is passed through the Hall sensor. In this mode, the current (flow of electrons) is evenly distributed across the chip. With an equal number of electrons at the chip edges, no voltage exists at the output. When the Hall sensor is placed in a magnetic field, the electron flow is forced to one side of the chip causing an imbalance in the electron concentration at the edges (Figure 4). This creates a voltage output that is detected and amplified to switch on an output transistor. On to the weather vane circuit — eight Hall sensors are placed around the vane shaft in positions of north, north-east, east, south-east, etc. A magnet is cemented to a PVC ring attached to the vane’s shaft. When the magnet comes in proximity to the Hall sensor, the transistor turns on and lights a corresponding LED on the circular display (Figure 5). The 1K resistor determines the brightness of the LED and the lower the resistance, the brighter the LED. Can’t find a suitable Hall-effect sensor (the Panasonic DN6849 from Digi-Key looks like a good bet)? You can substitute a reed switch in its place, as shown in the schematic to the top left. (See how serendipity works in hobby electronics? If you don’t have one part, lots of times another will work.) The weather vane sender can be connected to the LED display via a CAT 3 or CAT 5 communications cable. However, this can get expensive on long runs. A solution is to encode the eight inputs into BCD code and send it over the twisted pairs of cheap telephone wire (Figure 6), then decode it back to the original eight outputs. I hope you have learned something and had fun doing it at the same time. Have fun with your weather station! SILICON CONTROLLED SWITCH QCould you please explain the theory of operation of the new SCRs with turn-off gate and small current applications? — B. John 26 April 2006 ■ FIGURE 5 AWhat you are describing is a Silicon Controlled Switch (SCS) — sometimes called a tetrode thyristor. Like a Silicon Controlled Rectifier (SCR), the SCS is a four-layer device that goes into the avalanche mode when a positive voltage is placed on the cathode gate — and will continue to conduct even after the gate voltage is removed. The current flow is stopped by interrupting the current, a situation called natural commutation. The SCS, on the other
QUESTIONS & ANSWERS hand, has an additional gate connected to the anode side of the device. When a negative voltage is applied to the anode gate (or the anode is shorted to the cathode), it can turn off the SCS through forced commutation. A typical SCS circuit is shown in Figure 7. The four-layer SCS structure is equivalent to an NPN and a PNP ■ FIGURE 6 From Vane Sensors In0 In1 In2 In3 In4 In5 In6 In7 transistor configured in a forced feedback situation. When the ON pushbutton is pressed, voltage is applied between the cathode gate and the cathode, forward-biasing the lower transistor and turning it on. This, in turn, causes current to flow through the upper transistor’s base junction via R2 and turn it on. The SCS is now locked on, causing the motor to start and run — even after the ON switch is opened. Pressing the OFF push-button shorts the anode terminal to the cathode. This causes the upper transistor to lose its emitter current and break the current flow to the base of the lower transistor. The SCS now loses its lock on the anode/cathode current flow and turns off the motor. The SCS will remain in the off condition until the ON button is again pressed. SCS devices are not as widely used as once seemed possible. Their low use is likely due to the peak current that +5V Transmitter 1K Weather Vane 3-wire Transmitter/Receiver could be reliably turned off by the anode gate. Consequently, they are hard to find and often expensive. However, I was able to locate a small cache of the 3N86 at a reasonable price from Wholesale Electronics (www.weis d.com/store2/JSH3N86.html). It’s rated 65 volts at 200 mA, which will limit your experiments and applications. HIGH-POWER T-BIRD FLASHER QI am looking for a schematic for sequential lighting of tail lights in my ‘70 Mustang. Ford made these for their Cougar models. I would also like to control the speed of the sequence. +12V Could I use some small relays to ■ FIGURE 7 74HC148 I0 EO I1 I2 I3 I4 A0 I5 A1 I6 A2 I7 EI GS 1K Front Blinker TIP 110 Phone Wire 4017 Q5-9 Q9 Q8 Q7 Q6 Q5 CP1 Q4 CP0 Q3 Q2 MR Q1 Q0 74HC04 accomplish this? I have #1157 bulbs. — Jim Wiggins AI published a circuit that does exactly this in the March ‘06 issue, but it can only light an LED — not your eight-watt bulb. So, I went back to the drawing board and came up with the circuit in Figure 8. This design uses sensitive gate SCRs — like the Teccor TCR22-3 (available from Digi-Key; 800- 344-4539; www.digikey.com) — to light your existing bulbs. SCRs are tricky, in that once they are turned on, they remain on. Just what you want from the staircase pulses of the 4017 decade counter. But once all the lights are lit, they have to be turned off to start the 2N3906 1K Relay 100 100 100 Receiver +5V 74HC138 E1 Q0 E2 Q1 E3 Q2 Q3 Q4 A0 Q5 A1 Q6 A2 Q7 LED Display ■ FIGURE 8 100V 1.5A #1157 4011 4011 4011 4011 T-Bird Flasher 5uF 100K 1M April 2006 27
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