This time we describe the concept of touching a plate (or two plates separated by a small gap) and turning a circuit ON or OFF. A TOUCH PLATE is classified as a high impedance device (or high impedance circuit) as the effect of a finger will be detected by the circuit connected to the plate. To learn more about the concept of high impedance circuits, see Page 77 of our Basic Electronics course. If only a single plate is present, the circuit will actually be picking up "mains hum" from the finger. To prove this, take the project into an open space such as a large park and try the circuit. It will not work. If the plate has a signal on it (from an oscillator), the effect of your finger will be to remove the signal (or reduce its amplitude considerably) and a detecting circuit will be activated. If the circuit has two plates, it will be registering the resistance of your finger. If the circuit has 4 plates, it will use two to turn the circuit ON and two to turn the circuit OFF. There are a number of different types of TOUCH PLATES and different effects can be created by the circuit. 1. Touch a set of pads and the project turns on. When the finger is removed, the circuit turns off. The finger can touch the pads for any length of time. We also include the feature where the circuit extends the ON period, so the circuit stays on for a length of time after the finger is removed. This is shown in Circuits A. 2. Touch a set of pads fairly quickly and the project turns on. Touch the pads again for a short period of time and the circuit turns off. This is called the "Flip-Flop" effect. If the finger is kept on the pads, the circuit will turn on-off-on-off at a rate of about once per second. This is shown in Circuits B. 3. Touch one set of pads to turn the circuit on and another set of pads to turn the circuit off. This is shown in Circuits C. CIRCUITS A Here are a number of circuits that turn on a device when the touch-pad is touched. Putting a finger on the touch pads turns the top transistor ON and this transistor turns on the bottom transistor. When the finger is removed, the circuit consumes less than a microamp. However, if someone touches the touch pad, any added capacitance to ground reduces the ac drive at the FET's gate, and Q1 continuously conducts. The square-wave voltage applied to D1 decreases. The voltage on C4 drops below the logic threshold, and IC1B's output goes high. You can adjust R2 to set sensitivity and compensate for device-to-device variations in the FET's pinch-off voltage. The following circuit does not work. It uses a CD 4001
Try a NAND gate:
We need a component to allow the 4u7 to charge and make pins 5&6 LOW. The next diagram does this: CIRCUITS B The following circuits show a "flip-Flop" effect. The circuit changes state, each time the touch pads are touched. CIRCUITS C These circuits have two touch plates. One touch plate turns the circuit on and the other plate turns the circuit off.
The circuit above is available from Talking Electronics as a kit. The kit is called TOUCH SWITCH: A simpler version is shown below: Since the output of a CD 4011 is not capable of sinking or sourcing a high current, you can buffer the output of the gate with the third gate in the chip and wire it as an inverter. ON-OFF TOUCH SWITCH USING A 555 IC TOUCH PADSA touch Pad can be obtained from many different sources. The photos below show a touch pad obtained from a toy. Some of the very light touch buttons consist of a small carbon block mounted in silicon rubber and when the button is pressed, the carbon block touches the pad and reduces the resistance between the two interleaved tracks. This part of the circuit board can be cut away and used as a touch pad for the circuits in this discussion. The pads are already protected from corrosion and form a very good design for detecting a finger. The important feature of the pad is the number of interleaving fingers as this is equivalent to a pair of lines about 12cm long and when a finger is applied, the resistance between the lines drops to between 150k and 850k, depending on the pressure and moisture in the finger. HIGH IMPEDANCE CIRCUIT We have already said a touch pad is a high impedance device (circuit), but what does this mean and how does it work? We are going to explain why it must be a high impedance circuit. Below we have four different touch pad circuits. The supply voltage does not matter, however we have shown it as 6v. The main purpose of a touch pad is to reduce the voltage on the "output." Generally this must be15% - 25% of rail voltage to trigger the circuit. If we take the first circuit "A" and place a finger on the touch pad, the circuit becomes equivalent to two resistors in series. These two resistors form a voltage divider and the voltage on the output is in proportion to the value of the resistances. We will assume the resistance of the finger is 1M to make the discussion simple. The 5M resistor is not a standard value but s also used to make the discussion easy to understand. In the diagrams below, the output of the touch pad is 6v when nothing is touching the pad. When a finger touches the pad, the voltage drops to 1v. Without using mathematics, we can see the 5Meg resistor is in series with the 1Meg finger, making a total of 6Meg. This means 1v appears across each 1Meg and thus the output is 1v. From this we can see the "pull up" resistor must be as high as possible so the effect of a finger will reduce the output voltage of the pad to a low value. There is one other important factor to remember. The output of a touch pad must be connected to a high impedance input. The diagram below shows the gates and a "super-alpha" transistor. These all have a high impedance input. Why do we need a high impedance input? Suppose the circuit we are connecting to the touch pad has a low impedance. It will be equivalent to placing your finger on the touch pads. The output will go low and your finger will not be able to create a HIGH-LOW voltage change. The input impedance of a gate can be considered to be very high (greater than 10M). When the "super-alpha" pair is connected to the touch switch, the voltage on the "output" of the touch pad will not rise above 1.3v. This is due to the base-emitter junctions of the two transistors. The output of the super-alpha pair will be low. When a finger is placed on the touch pads, the output of the super-alpha pair will rise. An alternate circuit for connecting touch pads to a super-alpha pair is shown below: LATCH CIRCUITHere are two latch circuits using transistors. The first operates exactly the same as the 4-transistor Touch Switch above. It can be used with a touch pad. It's another "Building Block" to add to your collection. The second circuit operates in the same way. When the circuit is first turned on, both transistors are not conducting. As the input voltage increases to 0.65v, the BC 547 transistor turns on and this turns on the BC 557. The BC 557 is connected to the base of the BC 547 and it takes over from the input voltage. The two transistors turn each other on until both are fully turned on. The supply must the turned off to reset the circuit. That's why you need to know how to design circuits, so you don't over-design.See our "Spot The Mistake" article for more over-designed and incorrectly designed circuits. You learn more from other people's mistakes than anything else. USING A TOUCH SWITCH IN A PROJECT 1. DOORKNOB ALARMThe 74C14 (40106) is a hex Schmitt trigger IC with 6 gates that can be used for 6 different building blocks. Even though it has a "74" marking, it can be placed in a circuit with a voltage as high as 15v - all the other 74 series require a maximum of 5v for the supply. (More data on the 74C14 can be found in Chip Data eBook.) In the following circuit, the gates are used to detect the touch of a door knob and produce an output that goes HIGH for approx 1 minute. As the capacitance on pin 1 is decreased, the frequency of the oscillator increases and this makes it easier for the human body to absorb the signal. Try changing this value as well as the coupling 5p6 to 22p that connects the oscillator to the detecting gate between pins 3 and 4. The photo shows the device and magnet. The magnet holds a reed switch closed and when the two items are parted, the reed switch opens and sounds the alarm. The reed switch can be seen in the photo below. It is an uncovered reed switch consisting of two soft-iron strips that overlap slightly in the centre. When a bar magnet is brought near, the two strips become magnetised with each forming a north at the top and south pole at the bottom. This means the top strip has a south pole at its bottom and the lower strip has a north pole at its top. Since unlike poles attract, the two strips will touch each other when a bar magnet is present. The magnetic field of the bar magnet causes the two parts of the reed switch to become "magnetic." The side of the alarm showing Chime (Doorbell), Off and Alarm. See below for a link to these sounds. The underside of the alarm showing the COB module and the 4 pins from the transformer that drives the piezo diaphragm. To hear the "DoorBell" sound and "Alarm" sound, click HERE. or here: SOUND Open the reed switch so the Door-Knob circuit can operate the alarm. 2. TOUCH MOTOR CONTROL - by L. W. Brown, Burwood, Victoria, Australia. The following circuit is suitable for operating a12v motor such as on a display in a shop window. The 50mm x 50mm touch plate can be stuck to the inside of the glass and anyone placing their finger near the touch plate (on the outside of the window) will prevent the signal entering the charge pump section of the circuit and keeping the 10n charged. The circuit will take a few seconds before the 10n is discharged via the 10M and the motor will operate. 3. TOUCH-ON TOUCH-OFFThis circuit is an extension of the Door-knob Alarm presented above. It turns on an output when the Touch-Plate is touched very briefly and turns off the output when the plate is touched for a slightly longer period of time. As the capacitance on pin 1 is decreased, the frequency of the oscillator increases and this makes it easier for the human body to absorb the signal. Try changing this value as well as the coupling 5p6 to 22p that connects the oscillator to the detecting gate between pins 3 and 4. A reader had success when the capacitor on pin 1 was reduced to 5p6 as this increased the frequency of the oscillator to approx the maximum frequency of the gate and this high frequency was readily absorbed by the body when the touch plate was touched. TOUCH-ON TOUCH-OFF SWITCH It has also covered the concept of a HIGH IMPEDANCE CIRCUIT and FEEDBACK to keep a circuit stable in either of its two states. Even if you think you will never need a TOUCH SWITCH in a future project, the building blocks we have covered can be used in lots of different circuits and if you build them, you will have a much-better understanding of how they work. 26/5/10 |
Engineer Faisal Kamran Gondal
Search This Blog
Sunday, September 26, 2010
TOUCH SWICTH CIRCUIT
Friday, September 24, 2010
Subscribe to:
Posts (Atom)