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Design by BRANCO JUSTIC* Two servos are used to provide tilt and pan motion to this small CCD video camera. Now you can remotely control a camera while you watch the video monitor. PAN 58  Silicon Chip your BY Do you have a video security system involving a minia­ture CCD camera? How would you like to be able to remotely pan it from side to side and up and down while you watch the video monitor? This circuit uses two servos to do the job and draws no current at all while the camera is stationary. More and more people are finding uses for tiny CCD video cameras. They’re not just being used in routine security applica­ tions but they are being used around the home for watching young children, especially around swimming pools, in hospitals and so on. But most of these cameras would be fixed installations, so the view on the screen is always of the same room or whatever. Now it is possible to remotely pan the camera while you watch the monitor. In practice, the CCD camera is mounted as shown in our photos. These show a typical miniature CCD camera mount­ed in a small plastic case which is attached to a servo disc (ie; a round flange attached to the servo shaft). This first servo is then mounted on an angle bracket which is attached to a second servo disc. The first servo pans the camera up and down while the second servo pans it from side to side. Servo driver The servo control circuit is mounted in a plastic utility case with two knobs and a central button. Each knob controls a servo while the central button is labelled “Execute”. This is not a form of punishment but merely means that nothing happens to the servos unless the button is pressed. This has the effect of minimising servo wear and tear but more importantly, if the button is not pressed, the circuit is completely dead and so the battery (if battery power is used) is conserved. This approach to servo drive is quite novel but is practi­cal in this application. After all, you don’t want the servos drawing current while the camera remains pointed in a fixed direction. It might be left in this condition for hours or days at a time, so it makes sense to power the circuit only while the camera is actually being moved. You could use the servo control circuit in one of two ways. First, you might rotate the pots to set a new camera position and then push the “execute” button. The camera will then move to the new position and stop. Second, you might hold the “execute” button down while you twiddle the pots so that the camera moves exactly in sympathy with rotation of the pots. An ideal method would be to use a joystick potentiometer set from a Parts List 1 plastic utility case, 130 x 67 x 42mm 1 PC board, 46 x 60mm 2 servos 2 servo discs 1 9V, 10V or 12V DC plugpack 1 momentary contact pushbutton switch (S1) 2 100kΩ potentiometers (VR1, VR2) Semiconductors 1 74C14, 40106 hex Schmitt trigger (IC1) 1 TIP41C NPN power transistor (Q1) 3 BC548 NPN transistors (Q2, Q3,Q4) 1 6.2V 400mW zener diode (ZD1) 3 1N4148, 1N914 silicon diodes (D1,D2,D3) Capacitors 3 10µF 16VW PC electrolytic 2 .012µF MKT or greencap polyester 2 .01µF MKT or greencap polyester Resistors 2 1MΩ 2 68kΩ 4 10kΩ 3 2.2kΩ 2 1kΩ radio control transmitter but at the time of writing we had not been able to access a suitable joystick at a reason­ able price. Circuit description Fig.1 shows the circuit of the servo controller. It uses just one 74C14 CMOS hex Schmitt trigger inverter, CCD video camera REMOTE CONTROL January 1998  59 Fig.1: the circuit consists of two one-shot (monostable) pulse generators driven by oscillator IC1b. Most of the circuit is shut down until pushbutton S1 is pressed. The circuit and servos consume no power when not in use. a few diodes and transistors and not much else. There are two separate servo pulse generators, the first involving IC1c & IC1d and the second involving IC1e & IC1f. These are both driven by IC1b which is a free-running oscillator. Before we get too far ahead of ourselves though, let’s have a look at how the circuit starts itself. When power is first applied to the circuit, nothing hap­pens as far as the two servo outputs are concerned and the various Schmitt triggers do nothing. The output of the 5V regula­tor, comprising transistors Q1 & Q2, is also close to zero. Everything depends on IC1a and its output is close to zero This prototype board differs somewhat from the final version which has a screened parts overlay and solder masking. 60  Silicon Chip be­cause its input is held high due to the 1MΩ resistor and 10µF capacitor at pin 13. When pushbutton S1 is pressed, pin 13 of IC1a is pulled low and the 10µF capacitor is charged via the 2.2kΩ resistor, R2. Pin 12 of IC1a goes high and this does two things. First, it feeds a bias current to the base of Q2 via a 2.2kΩ resistor, R3. This develops 6.2V across zener diode ZD1 and allows Q2 and Q1 to work as a 5V regulator to provide power to the two servos and to transistors Q3 & Q4. At the same time, pin 12 of IC1a reverse-biases diode D1 and this allows IC1b to operate as a free-running oscillator, with its frequency set by the .01µF capacitor and 1MΩ resistor at its pin 1. It produces a square wave at about 60Hz. Now let’s look at the servo pulse generator involving IC1c & IC1d. This really operates as a one-shot to produce a single positive pulse with a duration set by the 100kΩ potenti-ometer VR1. Let’s look at what happens, in slow motion. First, each time the output of IC1b goes high, it pulls the input of IC1c, pin 11, high. This causes pin 10 to go low and this low signal is fed via the .012µF capacitor to pin 9 Fig.2: these scope waveforms show the servo signals from the emitters of Q3 & Q4. The pulse widths are varied by the poten­tiometers VR1 & VR2. of IC1d. Pin 8 of IC1d then goes high and stays high until the capacitor at pin 9 is charged via VR1 and the series 68kΩ resistor. This causes pin 9 to be pulled high to the point where pin 8 must go low. The result is a +12V pulse at pin 8 with a duration of between 1ms and 2ms (nominal), depending on the setting of VR1. This pulse is fed to Q3 which acts as a voltage level translator and buffer, changing the +12V pulse at pin 8 to a pulse with a nominal amplitude of +5V which is compatible with the servos. Exactly the same process happens with the other one-shot pulse generator comprising IC1e & IC1f. Each time the oscillator output of IC1b, pin 2, goes high, a positive pulse appears at pin 6 of IC1f and this is fed via transistor Q4 to the second servo. So both pulse wavetrains are synchronised to each other, as can be seen from the two scope waveforms shown in Fig.2. However, this whole process only lasts about 10 seconds which is more than enough for each servo to come to rest and stabilise at its new setting. After that time, the 10µF capacitor at pin 13 of IC1a will have discharged sufficiently via the shunt 1MΩ resistor to pull pin 13 high. This causes pin 12 to go low and this shuts down the 5V regulator and disables the oscillator involving IC1b via diode D1. Thus, the +5V rail to the servos and the servo pulse signals are killed and so the servos are stuck at their last position. In this condition the circuit draws negligible current. Note that as long as you hold Fig.3 (left): the wiring diagram for the dual servo controller. If you do not wish to use the power-saving feature, the pushbutton switch could be replaced by a wire link. Resistor Colour Codes ❏ ❏ ❏ ❏ ❏ ❏ No. 2 2 4 3 2 Value 1MΩ 68kΩ 10kΩ 2.2kΩ 1kΩ 4-Band Code (1%) brown black green brown blue grey orange brown brown black orange brown red red red brown brown black red brown 5-Band Code (1%) brown black black yellow brown blue grey black red brown brown black black red brown red red black brown brown brown black black brown brown January 1998  61 The PC board is mounted on the lid of the case and connected to the Pan and Tilt potentiometers via flying leads. Power comes from a DC plugpack supply. pushbutton S1 down the cir­cuit will continue to work but it will stop about 10 seconds after the button is released. If you want to have the circuit permanently powered, S1 could be a toggle switch or it could be linked across. Note: readers wanting a detailed description of the opera­tion of servo encoder and decoder circuitry should refer to the Radio Control articles by Bob Young in the November & December 1997 issues of SILICON CHIP. Construction All the components of the circuit, with the exception of the two potenti­ ometers and the pushbutton switch, are mounted on a small PC board measuring 46 x 60mm. The component layout is shown in Fig.3. Assembly is quite straightforward. Insert the PC pins first, followed by the resistors and diodes. Then insert the capacitors and the transistors. The CMOS IC should go in last. Note: there are positions on the supplied PC board labelled D4 and D5 but these diodes are not required for the circuit to work. The finished PC board is mounted in a plastic utility case and connected to the two potentiometers and push­ Where To Buy The Kit All the parts for this kit are available from Oatley Elec­tronics who own the design copyright. Their address is PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563; fax (02) 9584 3561. The prices are as follows: Complete kit for dual servo controller................................................$19.00 Servo kits.................................................................................$15.00 each DC plugpack......................................................................................$10.00 Pinhole or standard CCD video camera............................................$89.00 Camera box plus universal swivel bracket...........................................$4.00 62  Silicon Chip button switch via flying leads. When you have finished assembly, carefully check all your work against the circuit of Fig.1 and the wiring diagram of Fig.3. If everything is OK, apply +12V to the supply input and check voltages around the circuit. You should find +12V at pin 14 of IC1 and at the collectors of Q1 & Q2. No voltage should be present at the col­lectors of Q3 & Q5. Furthermore, pins 2, 3, 5, 9, 11 & 13 of IC1 should be high (ie, close to 12V) while pins 1, 4, 6, 8, 10 & 12 should be low (ie, close to 0V). When the pushbutton is pressed, pin 13 should go low, pin 12 will go high and the other pins of the IC will be at a voltage somewhere between high and low. The emitter of Q1 should be at +5V. The circuit will then revert to its original quiescent condition after about 10 seconds. Now connect your two servos, press the button again and you should be able to move both servos with their respective poten­tiometers. Having verified that the circuit works, you are ready to set up your camera and starting panning to your heart’s content. *Branco Justic is the Managing Director of Oatley Electronics.