START UP CONTROL

Description: A bidirectional three-layer trigger for a triac allows triggering on both half-cycles, with the activation point determined by a 1-megohm potentiometer setting. The triac's rating dictates the load size it can manage. This circuit combines two functions: it serves as both a passive cutoff device and a simulated car alarm. Pressing switch S1 while the key is in the RUN position disables the flasher, enabling the car to start. When the AC line twitch is closed, power is not delivered to the load until the line voltage crosses zero. Identical circuits manage each half of the AC cycle. A transistor turns on at 1.4 V, preventing the SCR from triggering until 0.013 ms (less than one-third of an electrical degree) after the next zero-crossing point. This configuration was referenced in "Efficient and Simple Zero-Crossing Switch" by A. S. Roberts and O. W. Craig in EDN|EEE Magazine, August 15, 1971.

The system is utilized to control a low-power transceiver for the 20-meter amateur band. A sidetone is generated from a simple relaxation oscillator using a GED-13-T programmable UJT. The sawtooth output is attenuated and fed into a two-transistor audio amplifier. Transmitter keying is accomplished through a series switch using a 2N4036 silicon transistor. An antenna relay applies +12 V to the appropriate transceiver stages during transmit (+12T) and receive (+12R) operations, facilitating the switching of the antenna between the transmitter power chain and the receiver RF amplifier. A DPDT relay with an 800-ohm, 12-V coil is recommended. The transmit/receive logic employs an A741C op-amp as a differential comparator. When the key is pressed, a 2.2-µF capacitor discharges, causing the op-amp output to switch to a high state and saturate a 2N3904 relay driver, activating the relay for transmission. Upon releasing the key, the capacitor begins to charge; at a 6-V threshold (approximately 0.5 seconds with a 220K timing resistor), the op-amp state changes again, opening the relay for receiving, as discussed in "Low-Power Single Band CW Transceiver" by W. Hayward in Ham Radio, November 1974.

When the peak-to-peak value of the input signal exceeds a preset reference voltage, slicer Q2 conducts, allowing Q4 to amplify the signal for input to a Schmitt trigger, which squares the signal. Q7 then delivers an output that modifies the fixed-gain amplifier to unity gain, effectively providing fast automatic gain control (AGC) for a monopulse radar amplifier, as described by W. W. Smith in "Fast AGC Amplifier Locks Monopulse Radar on Target Electronics."

The circuit is also developed for use with an AC sensor in one leg of a bridge configuration. The CA3094 IC is disabled during the negative half-cycles of the AC line. If the bridge becomes unbalanced, making pin 2 more positive than pin 3, the IC is off at the moment the AC line swings positive; pin 8 then goes high, triggering the triac into conduction. The triac remains conductive during the subsequent negative half-cycle due to energy stored in a 100-µF capacitor. An unbalanced bridge in the opposite direction does not trigger the triac, as noted in "Linear IC Principles, Experiments, and Projects" by E. M. Noll.

The circuit achieves a gain of 15 dB with a typical noise figure of 5 dB. The reverse gain control range is 25 dB, varying from a collector current of 1.5 mA to 20 µA, providing 20 dB of forward gain control, according to Texas Instruments Inc. in "Solid-State Communications." The collector voltage and current are maintained constant while the gain adjusts according to the base-2 current, resulting in a gain of 21 dB and a typical noise figure of 60 dB.

This design employs a composite of DC coupling for dark scenes and AC coupling for bright scenes, with AGC referenced to the back-porch (blanking level) rather than sync tips, which approaches an ideal compromise for automatic control of television picture quality, as discussed by L. Solomon in "New Tubes and Circuits for Consumer Electronics." The amplifier is integrated with nonlinear circuit elements to achieve a D versus log E characteristic that closely resembles that of positive color film being scanned.Bidirectional three-layer trigger for triac allows triggering on both half-cycles at point determined by setting of 1-megohm pot. Triac rating determines size of load that can be handled. - SCR Power Control Fundamentals, Motorola, Phoenix, AZ, 1971, AN-240, p 6. (View) Combining two functions in one, this circuit is both a passive cutoff device a nd a fake car alarm Depressing S1 with the key in the RUN position disables the flasher and allows the car to start. (View) When AC line twitch is closed, power is not applied to load until after line voltage next goes through zero.

Identical circuits control each half of AC cycle. Transistor turn-on at 1. 4 V prevents SCR from triggering until 0. 013 ms (less than one-third electrical degree) after next zero-crossing point. -A. S. Roberts and O. W. Craig, Efficient and Simple Zero-Crossing Switch, EDN|EEE Magazine, Aug. 15, 1971, p 46-47. (View) Used to control low-power transceiver for 20-meter amateur band. Sidetone is obtained from simple relaxation oscillator using GED-13-T programmable UJT. Sawtooth output is attenuated and applied to input of two-transistor audio amplifier. Transmitter keying is done with series switch using 2N4036 silicon transistor. Antenna relay applies +12 V appropriately to transceiver stages used during transmit (+12T) and receive (+12R), and provides switching of antenna between transmitter power chain and receiver RF amplifier. Use DPDT relay with 800-ohm 12-V coil. Transmit receive logic uses A741C opamp as differential comparator. When key is closed, 2. 2- F capacitor is discharged, making opamp output switch to high state and saturate 2N3904 relay driver, pulling in relay for transmit operation.

When key is released, capacitor begins to charge; at 6-V point (about 0. 5 s with 220K timing resistor), opamp changes state again and relay opens for receiving. -W. Hayward, Low-Power Single Band CW Transceiver, Ham Radio, Nov. 1974, p 8-17. (View) When peak-to-peak value of input signal exceeds preset reference voltage, slicer Q2 conducts, making Q4 apply amplified signal to Schmitt trigger for squaring. Q7 then delivers output ate that changes fixed-gain amplifier to unity gain to give effect of fast agc for monopulse radar amplifier.

- W. W. Smith, Fast AGC Amplifier Locks Monopulse Radar on Target Electronics, 36:39, p 34-36. (View) Developed for use with AC sensor in one leg of bridge. CA3094 is shut down on negative half-cycles of line. When bridge is unbalanced so as to make pin 2 more positive than pin 3, IC is off at instant that AC line swings positive; pin 8 then goes high and drives triac into conduction. Triac conduction is maintained on next negative half-cycle by energy stored in 100- F capacitor. Bridge unbalance in opposite direction does not trigger triac. -E. M. Noll, Linear IC Principles, Experiments, and Projects, Howard W, Sams, Indianapolis, IN, 1974, p 313-314.

(View) Gain is 15db. Typical noise figure is 5db. Reverse gain control range is 25 db from collector current of 1. 5 ma to 20 microamp. Provides 20 db of forward gain control. - Texas Instruments Inc. , Solid-State Comunications, McGraw-Hill, N. Y. , 1966, p 211. (View) Collector voltage and current are kept constant and gain is changed in accordance with base-2 current. Gain is 21 db and typical noise figure is 6O db, Texas Instruments Inc. , Solid-State Communications, McGraw-Hill, N. Y. , 1966, p 213. (View) Composite of d-c coupling for dark scenes and a-c coupling for bright scenes, with agc referenced to back-porch (blanking level) rather than to sync tips, approaches ideal compromise for automatic control of tv picture.

-L. Solomon, New Tubes and Circuits for Consumer Electronics, Electronics, 36:2, p 47-49. (View) Amplifier is used with nonlinear circuit elements to gel D-versus-log E characteristic approximating that of positive color film being scanned. When no signal is applied to grid of V1, all diodes