circuit wiring

Quiz-type game shows are increasingly becoming popular on television these days. In such games, fastest finger first indicators (FFFIs) are used to test the player’s reaction time. The player’s designated number is displayed with an audio alarm when the player presses his entry button. The circuit presented here determines as to which of the four contestants first pressed the button and locks out the remaining three entries. Simultaneously, an audio alarm and the correct decimal number display of the corresponding contestant are activated. When a contestant presses his switch, the corresponding output of latch IC2 (7475) changes its logic state from 1 to 0. The combinational circuitry comprising dual 4-input NAND gates of IC3 (7420) locks out subsequent entries by producing the appropriate latch-disable signal. Priority encoder IC4 (74147) encodes the active-low input condition into the corresponding binary coded decimal (BCD) number output.

The outputs of IC4 after inversion by inverter gates inside hex inverter 74LS04 (IC5) are coupled to BCD-to-7-segment decoder/display driver IC6 (7447). The output of IC6 drives common-anode 7-segment LED display (DIS.1, FND507 or LT542). The audio alarm generator comprises clock oscillator IC7 (555), whose output drives a loudspeaker. The oscillator frequency can be varied with the help of preset VR1. Logic 0 state at one of the outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby enabling the audio oscillator. IC7 needs +12V DC supply for sufficient alarm level. The remaining circuit operates on regulated +5V DC supply, which is obtained using IC1 (7805). Once the organiser identifies the contestant who pressed the switch first, he disables the audio alarm and at the same time forces the digital display to ‘0’ by pressing reset pushbutton S5. With a slight modification, this circuit can accommodate more than four contestants.

VU LED indicator is simpler and smaller than their analog, and very common in audio equipment. This version is based on National Semiconductor IC, and using the logarithmic version. Each LED operates with a 3dB difference from before, and the jumper is provided to allow dot or bar mode.


The circuit is completely conventional, and is based on application notes from National Semiconductor. The circuit is shown in the picture below and you can view it with a single IC and some discrete components. DC to the LED is almost filtered - C1 are included to ensure that the IC does not oscillate, and is not a filter cap.


This allows a higher LED current with lower dissipation than is the case if the DC is fully refined, and full smoothing would also require much larger capacitors. This will increase the size and cost of the project - especially important if you want to use in larger quantities that may occur with a mixer or analyzer.It could not be simpler. At the maximum level that you want to operate the equipment (as shown on the audio millivoltmeter or an oscilloscope with a signal applied), adjust VR1 so that the signal light to all the green LED (L1 is the most sensitive, and L10 showed the maximum level, so L1 to L8 should be lit ). If the input directly from the speaker output, an additional series resistor should be used in terminal "Aud" input to reduce the level. This can be determined by calculation (I leave it to you) or by experiment. As a guide, for 50W amplifier, an external resistance should be about 47k ohms.If you calibrate the meter for the power amplifier, set the output to a level below clipping. Adjust the level control until the LED turns on. In this way, if (L10) LED bulbs last when youre listening to music, youll know that you are very close to clipping, and volume should be reduced.

With the aid of an (old) moving coil instrument it is very little effort to make a simple voltmeter that, at a glance, indicates the status of a telephone line. Because the input impedance of this circuit is very high, there is no problem in having it permanently connected to the line, since it only draws a tiny amount of current. The schematic shows that the circuit consists of no more than a series resistor, a bridge rectifier and a moving coil meter. The value of the resistor depends on the sensitivity of the moving coil meter. In his prototypes, the author used old VU meters that require 250µA for full-scale deflection. A resistor value of 390 kΩ appeared to be optimal for these meters.


For a 100µA instrument, this resistor value will have to be increased to about 680 kΩ. The starting point, when selecting a resistor value is that when the telephone is not in use, the meter should deflect about 2/3rd of full scale. The amount of meter deflection indicates the three different states of the telephone line: 1. The deflection is very small: the line is in use (voltage 5 to 12 V). 2. The deflection is 2/3rd of full scale: the line is not in use (voltage typically 48 V). 3. Full-scale deflection: ring signal (60 to 90 V AC). Because the idle voltage and certainly the ring voltage are high enough to be dangerous, it is recommended that the circuit is constructed in such a way that it presents no hazard when touched.