Schematic Volt

A small audio test generator is very useful for quickly tracing a signal through an audio unit. Its main purpose is speed rather than refinement. A single sine-wave signal of about 1 kHz is normally all that is needed: distortion is not terribly important. It is, however, important that the unit does not draw too high a current. The generator described meets these modest requirements.

It uses standard components, produces a signal of 899 Hz at an output level of 1 V r.m.s. and draws a current of only 20 µA. In theory, the low current drain would give a 9 V battery a life of 25,000 hours. The circuit is a traditional Wien bridge oscillator based on a Type TLC271 op amp. The frequency determining bridge is formed by C1, C2 and R1–R4. The two inputs of the op amp are held at half the supply voltage by dividers R3-R4 and R5-R6 respectively.

Circuit diagram:


Resistors R5 and R6 also form part of the feedback loop. The amplification is set to about ´3 with P1. Diodes D1 and D2 are peak limiters. Since the limiting is based on the non-linearity of the diodes, there is a certain amount of distortion. At the nominal output voltage of 1 V r.m.s., the distortion is about 10%. This is, however, of no consequence in fast tests.

Nevertheless, if 10% is considered too high, it may be improved appreciably by linking pin 8 of IC1 to ground. This increases the current drain of the circuit to 640 µA, but the distortion is down to 0.7%, provided the circuit is adjusted properly. If a distortion meter or similar is not available, simply adjust the output to 1 V r.m.s. Since the distortion of the unit is not measured in hundredths of a per cent, C1 and C2 may be ceramic types without much detriment.

This electronic game is simulation of one-arm bandit game. Electronics hobbyists will find it very interesting. When toggle switch S1 is in ‘run’ position, all segments of 7-segment displays (DIS1 through DIS3) will light up. On turning toggle switch S1 from ‘run’ to ‘stop’ position, displayed digits will continue advancing and the final display is unpredictable. Thus the final number displayed in DIS1 through DIS3 is of random nature. The speed with which the number in 7-segment display keeps changing on flipping switch S1 from ‘run’ to ‘stop’ condition slowly decays before stopping with a random number display. To play this game, one has to obtain three identical numbers in displays DIS1 through DIS3.

The contestant would score 1 (one) point if he manages to get a final display of ‘000’, 2 points for getting ‘111’ display, 3 points for ‘222’,… and so on—up to ten points for ‘999’. He should try to score maximum possible points in fixed numbers of attempts (say, 20 to 25 attempts). Apart from using this circuit as a game for entertainment, one can use it as random number generator for any other application as well. The decay time with the given component values is around 15 seconds before the display could stop at a final random number.

Circuit diagram:
The circuit comprises clock oscillator built around NE555 timer IC4, three-stage clock pulse counter built using three CD4033 ICs (IC1 to IC3), and three 7-segment LED displays (DIS1 to DIS3). In clock oscillator circuit, NE555 timer IC4 is used in a similar way as a free-running astable multivibrator, the only difference being the additional capacitor C1 introduced between pin No. 7 of IC4 and junction of resistors R22 and R24. When toggle switch S1 is in ‘run’ position, both terminals of capacitor C1 are shorted by switch S1 and timer IC4 works as a free-running astable multivibrator. The operating frequency is in the vicinity of 35 kHz, determined by the value of timing components.

When toggle switch S1 is flipped from ‘run’ to ‘stop’ position, capacitor C1 is introduced in the discharge path of pin No. 7 of IC4 and junction of resistors R22 and R24. At the same time, capacitor C4 comes in parallel with timing capacitor C3 to change the operating frequency of the astable from around 35 kHz to around 65 Hz. Now capacitor C1 slowly starts charging as it is connected in the discharge path of the timing capacitors C3 and C4. The clock frequency of IC4 gradually reduces and after 15 seconds, when capacitor C1 is sufficiently charged, the oscillating frequency gradually drops and finally it stops oscillating. Thus, pin 3 of IC4 becomes low.

Second part of the circuit comprises three cascaded ICs, IC1 through IC3 (CD4033 decade upcounter cum 7-segment decoder). In conjunction with three 7-segment displays (DIS1 to DIS3), these form a 3-digit clock counter. The clock counting speed is dependant upon the clock pulse frequency of IC4. It is connected to clock input pin 1 of IC1 while chip enable pin 2 of IC1 to IC3 are held low. Thus all clock counter ICs advance by 1 for every positive clock transition. Reset pin 15 of all counter ICs is held low through resistor R25. Thus reset facility is not used in this circuit. Due to persistence of vision, one can not distinguish 0-9 counting in DIS1 to DIS3 when the clock frequency is high.

All 7-segment displays appear to show digit 8, while the red LED1 remains lit continuously, indicating clock counter is in running condition. On sliding toggle switch S1 from ‘run’ to ‘stop’ position, the counting speed of individual digits falls immediately due to the clock frequency changing to around 65 Hz. Now, the counting speed will be 65 Hz for DIS3, 6.5 Hz for DIS2, and 0.6 Hz for DIS1. This speed of individual digit counting slowly decays, until the counter stops and LED1 stops blinking, and the final count (random numbers) are displayed in DIS1, DIS2, and DIS3.