| Introduction |
These circuits was found in the RCA Hobby Circuit Manual, 1970. It's only right that a keyer built with discrete parts, rather than integrated circuits, be included with the other keyers. The first keyer emulates a "Bug", or Semi-Automatic Key. Normally, with a "Bug", Dots are formed with a vibrating arm, and the Dashes are formed manually. The first keyer operates in a similar manner. The second keyer automates the Dots and the Dashes.
The article provides some templates for building the Automatic Keyer. It would be interesting just to build it up on a bread board. Substitution with more up to date parts shouldn't be a problem.
The descriptions below are directly from the Hobby Circuit Manual. They are only here for clarity as, magazine article copies are often difficult to read.
| Semi-Automatic Morse-Code Keyer |
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Printable Image of the Schematic
- Semi-Automatic Morse-Code Keyer
The Semi-Automatic Morse-Code Keyer or "bug" generates a single dot or a series of dots, depending upon how long the paddle-key is depressed; the dash must be made manually. The rate at which dots are generated can be varied.
The dot repetition rate is determined by R4, C1, and the SPEED potentiometer R20. These components control the regenerative switch consisting of transistors Q1 and Q2. This switch has a very high impedance before it is triggered and a very low impedance afterward. When the paddle-key is moved to the dot position, the current applied to the base of Q3 turns it on and permits C1 to begin charging through the emitter of Q2. At the same time, Q2 turns on and triggers the regenerative switch into conduction. As capacitor C1 charges, the emitter of Q2 becomes more and more positive until the regenerative switch is cut off. When cutoff occurs, the impedance of the regenerative switch becomes very high and C1 is forced to discharge through R4 and the speed potentiometer R20. As the charge on C1 decreases, the emitter of Q2 becomes less positive and the regenerative switch begins to conduct again. This process repeats itself as long as the paddle-key is held in the dot position. The polarity of the regenerative switch in conduction is such that a negative pulse is applied to the base of transistors Q4 and Q5 in the flip-flop. This negative pulse is sufficient to turn on transistor Q4. Q5 turns off automatically as a result of normal flip-flop action. When Q5 is off, its collector voltage is applied to Q6 through R16, and Q6 turns on. Current through Q6 activates Q7 which, in turn, closes the output relay. Diode CR4 is placed across the relay to protect Q7 from the high voltage inductive discharges which occur when current to the relay coil is interrupted and its coild field collapses.
When the paddle-key is released from the dot position with Q4 off (i.e., when the paddle-key is released at the end of a dot), Q3 turns off and interrupts the C1 charging path, with the result that the regenerative switch pulses that cause the dots are stopped. When the paddle-key is released from the dot position with Q4 on (i.e., when the paddle-key is released in the middle of a dot), Q3 continues to conduct because its base current continues to flow through Q4. The regenerative switch pulses once more to complete the dot cycle. Dot-cycle completion is accomplished when the final regenerative switch pulse returns the flip-flop to its original state and turns Q4, and consequently, Q3, off.
If instead of batteries a power supply is used to power this circuit, the 1.5 volts needed (shown as an input at circuit-board terminal No. 6) can be obtained from the drop across the two rectifiers CR5 and CR6 connected in series, as shown in the schematic.
When the paddle-key is in the dash position, the relay is not under the control of a transistor, but operates directly.
| Page 1 Semi-Automatic Morse-Code Keyer |
| Automatic Morse-Code Keyer |
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Printable Images of the Schematic
- Page 1 - Automatic Morse-Code Keyer
- Page 2 - Automatic Morse-Code Keyer
The fully automatic keyer produces either dots or dashes continuously for as long as the paddle-key is held in the dot or dash position. The speed of the dots and dashes can be varied to suit the operator. The keyer circuit is composed of a a number of the building blocks described in the section on General Circuit Considerations: the pulser or clock, the flip-flop, and the lamp driver. The 12-volt supply is needed to power the keyer; eight flashlight batteries in series or a 12-volt supply such as that described in Circuit No. 2 may be used.
Circuit Operation
The schematic diagram for the fully automatic keyer are shown below. The dot or dash repetition rate of the keyer is determined by SPEED control potentiometer R29; the potentiometer controls the frequency of the pulser or clock oscillator consisting of transistors Q1 and Q2. When the paddle key is moved to the dot position (i.e., when terminals 8 and 9 on the circuit board are connected), a current is transmitted to the base of Q3, this current turns Q3 on. Q3 in turn activates the regenerative switch consisting of Q1 and Q2 and permits C1 to begin charging through the emitter of Q2. As capacitor C1 charges, the emitter of Q2 becomes more and more positive until Q2 is cut off. When cutoff occurs, the total impedance of Q1 and Q2 becomes very high and C1 is forced to discharge through R4 and the SPEED control potentiometer R29. As the charge on C1 decreases, the emitter of Q2 becomes less positive and transistors Q1 and Q2 begin to conduct again. This process repeats itself as long as the paddle-key is held in the dot or dash position. Q1 and Q2, when in conduction, produce a negative pulse that is applied to the bases of transistors Q4 and Q5 in the flip-flop. This negative pulse is sufficient to turn off transistor Q5; Q4 is turn on automatically as a result of normal flip-flop action. When Q5 is off, current is conducted through R12, CR10, and R27; this current turns Q9 on. Current through Q9 activates Q10 which, in turn, energizes the output relay.
The dash flip-flop composed of transistors Q6 and Q7 is held inoperative during the dot cycle by the clamping transistor Q8 which is held in the conductive state by current through R17 and R16. Rectifier CR11 is placed across the relay to protect Q10 from the high voltage pulse produced when current to the relay is interrupted and its coil field collapses.
When the paddle-key is released from the dot position with Q4 off (i.e., when the paddle-key is released during a space at the end of a dot or a series of dots), Q3 turns off and the oscillator pulses that cause the dots are no longer generated. When the paddle-key is released from the dot position with Q4 on (i.e., when the paddle-key is released in the middle of a dot), Q3 continues to conduct and permits the oscillator pulse to complete the dot cycle. This last pulse turns Q4, and consequently Q3, off, and the oscillator pulses cease.
A dash or series of dashes is produced when terminals 7 and 8 are connected (i.e., when the paddle-key is moved to the dashcomic_nb position). Under this condition Q3 is turned on by a signal applied to its base through R7 and CR7. At the same time Q8 is turned off by grounding of its base through CR8. The first pulse from the clock oscillator sets both the dot and dash flip-flops to the output state. Q3 receives a base signal not only from the dash flip-flop through CR2 and the dot flip-flop through CR1. Q9 receives a dash signal from either the dash or dot flip-flop through their respective diodes CR9 or CR10. The second pulse from the oscillator sets the dot flip-flop to the no-output state but does not disturb the dash flip-flop, and Q9 remains in conducting state. The third pulse sets the dot flip-flop to the output state, and Q9 remains conductive. When a fourth pulse is developed, both flip-flops are in the no-output state and Q9 is turned off. If at this time the paddle-key is in the neutral or middle position (circuit board terminals 7 and 8 disconnected), Q3 is also turned off and the system returns to its quiescent state. If the key is still in the dash position, the cycle repeats. Fig. 187 shows the voltage and current wave forms at selected points in the circuit. Relay current during a single dash sycle flows for a time equal to three dots and is cut off for a period equal to one dot.
| Page 1 Automatic Morse-Code Keyer |
| Page 2 Automatic Morse-Code Keyer |
| Power Supply |
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Printable Image of the Schematic
- Table IV
- Power Supply Schematic
The Hobby Circuit Manual suggests the following power supply circuits for the keyers listed above. The drawings were originally separate, but I combined them into one drawing. So the schematic contains two possible Transformer-Rectifier Stages, Full Wave and Full Wave Bridge, and two possible regulator stages, Series Regulator and Shunt Regulator. The power supply suggestions work together with the Table IV. Some of the descriptions below is from the manual, however, I have modified it slightly to match my drawings.
| Table IV Fixed Power Supply Design Chart | ||||||
|---|---|---|---|---|---|---|
| Transformer-Rectifier Stage | Regulator Circuit | |||||
| DC Output Voltage |
T1 Ckt.49(A) |
T1 Ckt.49(B) |
C1 (min) (µF/Volts) |
Circuit Type |
CR Voltage Ratings (V) |
R1 (Ω/Watt) |
| 3 VDC | 12.6 VAC | 6.3 VAC | 2500µF/10VDC | shunt | 3 forward biased RCA SK 3020's in series | 5Ω/5W |
| 4-1/2 VDC | 12.6 VAC | 6.3 VAC | 2500µF/10VDC | shunt | 3.3 VDC | 5Ω/5W |
| 6 VDC | 20 VAC | 10 VAC | 4000µF/15VDC | shunt | 4.7 VDC | 5Ω/5W |
| 9 VDC | 30 VAC | 15 VAC | 4000µF/15VDC | series | 10 VDC | 820Ω/1/2W |
| 10 VDC | 30 VAC | 15 VAC | 4000µF/25VDC | series | 11 VDC | 680Ω/1/2W |
| 12 VDC | 30 VAC | 15 VAC | 4000µF/25VDC | series | 13 VDC | 330Ω/1/2W |
| 15 VDC | 40 VAC | 20 VAC | 2500µF/50VDC | series | 16 VDC | 680Ω/1/2W |
| 18 VDC | - | 22.5 VAC | 2500µF/50VDC | series | 10 VDC and 9.1 VDC in series | 1000Ω/1/2W |
| 20 VDC | - | 28.5 VAC | 2500µF/50VDC | series | 11 VDC and 11 VDC in series | 470Ω/1/2W |
| 29 VDC | - | 38 VAC | 2500µF/50VDC | series | 15 VDC and 15 VDC in series | 1200Ω/1/2W |
| 35 VDC | - | 40 VAC | 2500µF/75VDC | series | 36 VDC | 680Ω/1/2W |
Your choice of Transformer-Rectifier Stage depends on your choice of transformer. You can see from Table IV that, the output voltage and number of diodes is dependent on the type of secondary. If you have a center-tapped secondary, you can use Ckt. 49(A) and only two rectifier diodes. Wheras, if the secondary is not center-tapped, four rectifier diodes, as in Ckt. 49(B), would be required. Also note that the required secondary voltage for Ckt. 49(A) is twice the secondary voltage for Ckt. 49(B).
For example, if you require a power supply with 12 VDC output, you can choose to use a 30 VAC transformer with a center-tapped secondary with just two rectifier diodes, Ckt. 49(A). Or, a 15 VAC transformer with four rectifier diodes, without a center-tapped secondary, Ckt. 49(B). Either one will provide the necessary DC Output Voltage for the regulator circuit.
Table IV suggests the Series Regulator for any DC voltage of 9 Volts and above.
The power supplies are suitable for use with the circuits in this Manual and for many other applications. The specific supply used is determined by the power requirements of the intended application. The maximum output current of any of these supplies is 1 ampere.