IC Timer Tester
The objective of this construction project is to build simple and easy to use test gadget that can be used to identify good and bad IC TIMERS from a given lot. Like opamps, an IC timer is another widely used electronic device and it finds its place in almost every project of hobbyists’ interest. This gadget not only enables you test the IC timer 555, it will also give you an opportunity to appreciate the tremendous functional potential of the various terminals of this truly versatile IC. Most of the circuits appearing in various magazines or books to test IC timers do the same by wiring the IC in one particular configuration only. Testing the IC timer with the gadget being described here not only tests the IC in all basic configurations, it practically tests the functionality of each and every terminal of the timer with the result that a timer declared fit by this gadget is healthy in the true sense and is certainly going to stand by you in whatever mode or configuration you may try it. Also, if you clearly understand the capabilities of this gadget and theory responsible for these features, you will be in a position to think of many more different modes in which you can use an IC timer. And moreover, it is going to be an excellent project for the students at diploma or an equivalent academic level.
CIRCUIT DESCRIPTION
The two basic configurations in which the IC timer 555 can be used are the (i) Astable mode of operation and (ii) Monostable mode of operation. When the DPDT switch (SW2) is in position 1-1, the timer under test automatically gets wired as a monostable multivibrator. In this case, the monoshot can be triggered by the microswitch (SW1). The Debouncing Circuit constituted by the two NAND gates of IC-1 produces a clean rectangular pulse from the pulse produced by pressing the microswitch. Resistor R3, Capacitor C1 and the Diode D1 ensure that the trigger terminal of timer IC 555 (pin-2 is the trigger terminal) gets the desired Vcc-to-ground trigger pulse. This differentiator circuit also ensures that the width of the trigger pulse is less than the expected monoshot output pulse. The monoshot output pulse width here depends upon the resistance offered by series combination of R8 and potentiometer P2 and the capacitor C4. These values have been so chosen that even for the minimum output pulse width, the trigger pulse width (trigger pulse width depends upon the differentiator time constant) remains less than that. Remember! the trigger pulse width to be less than the expected monoshot output pulse width is an essential requirement of this mode of operation.
When the DPDT switch is in position 2-2, the timer gets connected in the astable mode of operation. The output is a pulse train with the HIGH time determined by the total resistance offered by the series combination of R8, R9, P2 and capacitor C4 whereas the LOW time is determined by resistor R9 and capacitor C4.
The RESET terminal of IC timer (pin-4) should be tied to Vcc in the normal circumstances. More precisely, voltage at pin-4 should be greater than 0.8V. A voltage of less than that resets the output. Whether you have connected the timer in the monoshot or astable mode of operation, the output goes LOW the moment you bring the RESET terminal below 0.8V.
The CONTROL TERMINAL (pin-5) can be used to change the HIGH-time or the ON-time of the output pulse train in the astable mode and the pulse width at the output in the monoshot mode by applying an external voltage. This external voltage basically changes the reference voltage levels of the comparators inside the IC. The levels are set by three identical resistors of usually 5K inside the IC connected from Vcc to ground setting the levels at 2/3 (Vcc) for pin-5 and 1/3 (Vcc) for pin-2. These levels can be changed by connecting an external resistance from pin-5 to ground. Resistor R10 and potentiometer P3 have been connected for the same purpose.
The pulse width in the monoshot mode is given by:
1.1x (total charging resistance) x (charging capacitance)
This expression is valid when there is no external resistance connected from pin-5. The pulse width can be reduced by connecting an external resistance.
The HIGH and LOW times in the astable mode are given by:
HIGH-time = 0.69 x (charging resistance) x (charging capacitance)
LOW-time = 0.69 x (discharge resistance) x (capacitance)
Again the expressions are for no external resistance from pin-5. The HIGH-time can be made to decrease by connecting an external resistance from pin-5 to ground.
The circuit operates from +9V batttry which makes the gadget portable.
The test gadget can thus be used to carry out the following tests with given timer IC 555:
- The timer IC can be checked in the astable configuration.
- The timer IC can be checked in the monostable configuration.
- The capability of the RESET terminal to override all functions and rest the output to LOW can be checked.
- The function of the control terminal to change the ON-time or the HIGH-time of the output waveform in case of astable mode of operation and the output pulse width in case of monostable mode of operation can be verified.
CONSTRUCTION GUIDELINES
The components’ layout and the PCB layout as seen from component side are respectively shown in Figs.5.2 and 5.3. However the project can also be assembled on a general purpose PCB.
| Parts List | Specification |
| Resistors | |
| R1, R2 | 22K, 1/4W |
| R4, R5, R11 | 3.3 K, 1/4W |
| R6 | 4.7K, 1/4W |
| R7 | 47 ohms, 1/4W |
| R8 | 150K, 1/4W |
| R9 | 47K, 1/4W |
| R10 | 1K, 1/4W |
| R3 | 10K, 1/4W |
| Potentiometers | |
| P1 | 1K (preset) |
| P2, P3 | 100K (preset) |
| Capacitors | |
| C1, C3 | 0.01 μ F (ceramic disc) |
| C2, C6, C7 | 0.1 μF (ceramic disc) |
| C4 | 10μF, 25V (electrolytic) |
| C5 | 100μF, 25V (electrolytic) |
| Semiconductor Devices | |
| Diodes D1 to D4 | 1N4001 or equivalent |
| LED-1, LED-2, LED-3 | Preferably of different colours |
| IC-1 | CD 4011B |
| Miscellaneous | |
| 1. 8-pin Dual-in-Line IC Socket | |
| 2. Fuse with holder, 1 Amp. | |
| 3. Switch SW1 (Microswitch) | |
| 4. Switch SW2 (DPDT switch) SW3 (Plains ON/OFF switch) | |
| 5. Transformer : 14-0-14, 500mA Mains transformer | |
| NOTE: All resistors mentioned above are of carbon film or composition variety. | |
TESTING GUIDELINES
1. Insert the IC under test in the socket.
2. Set the DPDT switch in position-T-1.
3. Switch on the power supply.
4. LED-1 glows if the IC is healthy. LED-1 glows because the IC is presently wired as a monoshot and in the absence of any trigger, the output is LOW.
5. Give trigger pulse by pressing and releasing the switch SW1 once. You will observe the LED-1 getting extinguished and inturn the LED-2 becoming ON. This confirms that the output of the monoshot has gone HIGH. You will observe the LED-2 going OFF and LED-1 again becoming ON after some time. This will happen when the monoshot output goes LOW again. Vary the preset P2 so as to change the resistance offered by the pot. Trigger the monoshot again. You will find that the LED-2 glows this time for a longer or a smallertime period depending upon whether you increased or decreased P2 resistance. While carrying out this step-4 test, keep the presets P1 and P3 in the maximum resistance position.
6. Trigger the monoshot again and before the expected HIGH-time is over, quickly decrease preset P1 resistance so as to bring the voltage at pin-4 below 0.8V. You will observe the output to go LOW (indicated by a glowing LED-1 and an extinguished LED-2).
7. Set preset P1 resistance again in the maximum resistance position. Set the preset P3 in the minimum resistance position. Trigger the monoshot. You will observe the monoshot going HIGH foratime period that is much less than what is dictated by the resistance offered by the series combination of R8 and preset P2 and capacitor C4. Infact for a fixed setting of this series combination, the output pulse width can be observed to vary for different values of P3 resistance by triggering the monoshot several times, once for each setting of P3.
8. Set the DPDT switch in position 2-2. The LEDs 1 and 2 will start glowing alternately with the timings determined by resistances in charge and discharge paths. The timer IC is now in astable mode.
9. The functions of RESET and CONTROL pins can be seen in this configuration too in a similarfashion like the one discussed above in the case of monoshot configuration.
IDENTIFYING THE PINS
1. The pin connection diagram of CD 4011B (which is a QUAD 2-input NAND) is shown in Fig.5.4.
TESTING GUIDELINES
|
2. 3. 4. |
1. Insert the IC under test in the socket.
Set the DPDT switch in position-T-1.
Switch on the power supply.
LED-1 glows if the IC is healthy. LED-1 glows because the IC is presently wired as a monoshot and in the absence of any trigger, the output is LOW.
|
5. |
Give trigger pulse by pressing and releasing the switch SW1 once. You will observe the LED-1 getting extinguished and inturn the LED-2 becoming ON. This confirms that the output of the monoshot has gone HIGH. You will observe the LED-2 going OFF and LED-1 again becoming ON after some time. This will happen when the monoshot output goes
Written by arjun on December 11th, 2009 with
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