Isolated Fuse Fail Indicator

This circuit uses standard components and shows a method of indicating the fuse status of mains powered equipment while providing electrical isolation from the mains supply.

Circuit diagram:

Isolated Fuse Fail Indicator Circuit Diagram

A standard miniature low power mains transformer  (e.g. with an output of around 6 V at 1.5 VA) is used as a ‘sense’ trans-former with its primary winding (230 V) connected across the equipment’s input fuse so that when the fuse blows, mains voltage is applied to the transformer and a 6 V ac output volt-age appears at the secondary winding. The 1N4148 diode rectifies this voltage and the LED lights to indicate that the fuse has failed.

The rectified voltage is now connected to an RC low-pass filter formed by the 10 kΩ resistor and 100 nF capacitor. The resulting positive signal can now be used as an input to an A/D converter or as a digital input to a microcontroller (make sure that the signal level is within the microcontroller input voltage level specification). The 1 MΩ resistor is used to discharge the capacitor if the input impedance of the connected equipment is very high.

As long as the fuse remains intact it will short out the primary winding of the ‘sense’ transformer so that its secondary out-put is zero.

Read More

Yes-No Indicator Has Zero Standby Current

This circuit produces a random "Yes" or "No" with a single button press - indicated by the illumination of a red or green LED. The circuit has two advantages over similar circuits. First, it uses just a single momentary contact pushbutton, so no on-off switch is required. When the pushbutton is pressed, an oscillator comprising the 10nF capacitor and 22kΩ resistor at pins 1 & 2 is almost immediately stopped by FET Q1, which pulls the oscillator's timing capacitor to the positive rail. However, the 220nF capacitor and 470kΩ resistor in the gate circuit of Q1 introduce a tenth of a second's delay, so that about 250 oscillations take place before the clock is stopped.

Due to variations in charge on the circuit's capacitors, as well as voltage and temperature variations, and the unpredictability of when the pushbutton will be pressed, randomness is assured. The circuit has a high degree of randomness because it takes advantage of a near-perfect complementary square waveform at pins 10 and 11 of the 4047 IC. The oscillator frequency (available at pin 13) is passed through an internal divide-by-2 circuit in the 4047. This appears at pin 10 (Q), and is inverted at pin 11 (Q-bar), thus assuring a near perfect 50:50 duty cycle for the two LEDs.

Yes-No Indicator Circuit Diagram

Note:
However, that the "impartiality" of the circuit is partly contingent on the value of the 10nF capacitor and on a reasonably equal current flow through both LEDs. Over five trials, the Yes-No Indicator scored 142 Yes, 158 No, with Yes falling behind No in the fourth trial. Because the circuit only works while switch S1 is pressed, standby current is zero, therefore a miniature 12V battery may be used to power it. In this case the circuit could be used thousands of times before the battery would run flat. The circuit has a further potential use. If the LEDs are omitted and a piezo (capacitive) sounder is wired directly to pins 10 and 11, it will produce a loud beep when equipment is turned on, and will continue to draw less than 0.5mA until it is switched off. The frequency of the beep may be changed by altering the value of the 10nF capacitor and its duration by altering the value of the 220nF capacitor.

Author: Thomas Scarborough - Copyright: Silicon Chip Electronics

Read More

Maximum Minimum Voltage Indicator

This circuit indicates which of three voltages in the range from about about -4V to about +4V - at A, B and C - is the highest by lighting one of three indicator LEDs. Alternatively, it can be wired to indicate the lowest of three voltages or to indicate both the highest and lowest voltages. Op amps IC1a, IC1b & IC1c are wired as comparators, while the three indicator LEDs and their series 1kO current limiting resistors are strung across the op amp outputs to implement the appropriate logic functions.

 Maximum Minimum Voltage Indicator Circuit Diagram

For example, LED A will light only when pin 8 of IC1c is low (ie, A greater B) and pin 7 of IC1b is high (ie, A greater C). Similarly, LED B will light only when pin 8 of IC1c is high (ie, B greater A) and pin 1 of IC1a is low (ie, B greater C). LED C works in similar fashion if the voltage at C is the highest. Note that if all the LEDs and their parallel 1N4148 diodes are reversed, the circuit will indicate the lowest of the three input voltages. And if each 1N4148 diode is replaced by a LED, the circuit will indicate both the highest and lowest inputs.

Author: Andrew Partridge - Copyright: Silicon Chip

Read More

Water Level Indicator Circuit

Simple, two-wire, remote monitoring unit, Three-LED level display, 9V battery powered

The whole project was developed on a friend's request. Its purpose was to remotely monitor the water-level in a metal tank located in the attic by means of a very simple control unit placed in the kitchen, some floors below.

Mains requirements were:

1. No separate supply for the remote circuit
2. Main and remote units connected by a thin two-wire cable
3. Simple LED display for the main unit
4. Battery operation to avoid problems related to mains supply and water proximity
5. As the circuit was battery operated a low current consumption was obviously welcomed

The very small remote unit is placed near the tank and measures the water level in three ranges by means of two steel rods. Each range will cover one third of the tank capacity:

• Almost empty - signaled by means of a red LED (D3) in the control unit display
• About half-level - signaled by means of a yellow LED (D2) in the control unit display
• Almost full - signaled by means of a green LED (D1) in the control unit displa
  

Circuit diagram:

Water-level Indicator Circuit Diagram

Circuit operation:

When the water-level is below the steel rods, no contact is occurring from the metal can and the rods, which are supported by a small insulated (wooden) board. The small circuit built around IC1 draws no current and therefore no voltage drop is generated across R5. IC2A, IC2B and Q1 are wired as a window comparator and, as there is zero voltage at input pins #2 and #5, D3 will illuminate. When the water comes in contact with the first rod, pin #13 of IC1 will go high, as its input pins #9 to #12 were shorted to negative by means of the water contact. Therefore, R4 will be connected across the full supply voltage and the remote circuit will draw a current of about 9mA. 

This current will cause a voltage drop of about 0.9V across R5 and the window comparator will detect this voltage and will change its state, switching off D3 and illuminating D2. When the water will reach the second rod, also pin #1 of IC1 will go high for the same reason explained above. Now either R3 and R4 will be connected across the full supply voltage and the total current drawing of the remote circuit will be about 18mA. The voltage drop across R5 will be now about 1.8V and the window comparator will switch off D2 and will drive D1. The battery will last very long because the circuit will be mostly in the off state. Current is needed only for a few seconds when P1 is pushed to check the water-level and one of the LEDs illuminates.

Parts:

R1 = 15K 1/4W Resistors
R2 = 15K 1/4W Resistors
R3 = 1K 1/4W Resistors
R4 = 1K 1/4W Resistors
R5 = 100R 1/4W Resistor
R6 = 47K 1/4W Resistor
R7 = 3.3K 1/4W Resistors
R8 = 3.3K 1/4W Resistors
R9 = 2.7K 1/4W Resistors
R10 = 15K 1/4W Resistors
R12 = 15K 1/4W Resistors
R13 = 3.3K 1/4W Resistors
R14 = 2.7K 1/4W Resistors
R15 = 2.7K 1/4W Resistors
D1 = 3mm Green LED
D2 = 3mm Yellow LED
D3 = 3mm Red LED
C1 = 470nF 63V Polyester or Ceramic Capacitor
J1 = Two ways output sockets
J2 = Two ways output sockets
P1 = SPST pushbutton
B1 = 9V PP3 Battery
Q1 = BC547 45V 100mA NPN Transistor
IC1 = 4012 Dual 4 input NAND gate IC
IC2 = LM393 Dual Comparator IC
Two steel rods of appropriate length

Notes:

• The two steel rods must be supported by a small insulated (wooden) board
• IC1 and R1-R4 are mounted on a small board placed near or on the steel rods support
• The two-wire cable connecting the remote circuit board to the main control board, i.e. J1 to J2, can be of any size and type (preferably thin for obvious reasons). It can be very long, if necessary.
• The circuit can be used also with non-metal tanks, provided a third steel rod having the height of the tank will be added and connected to pin #7 of IC1, R3, R4 and J1.
• The 4012 chip was chosen because it contains two gates and was at hand, but you can use two of the gates contained into 4001, 4011, 4093, 4049, 4069 etc. chips, provided all inputs of each gate are tied together and all inputs of unused gates are connected to the positive rail, leaving output pins open.

Read More

Modem Off Indicator

 Modem Off Indicator Circuit Diagram. The modem off indicator is intended especially for serious Internet surfers. It will be seen that the circuit of the indicator cannot be much simpler, or there might be nothing left. In spite of its simplicity, it may prove to be a cost-saving device, since it shows at a glance whether the telephone line is free again after the modem has been used. This obviates high telephone charges in case for some reason the modem continues to operate. The circuit depends on the fact that there is a potential of about 40 V on the telephone line when it is not busy. This voltage drops sharply when a telephone call is being made. If, therefore, the circuit is linked to telephone terminals a and b, the lighting of the green LED shows that the line is not busy in error.

 Modem Off Indicator Circuit Diagram

The bridge rectifier ensures that the polarity of the line voltage is of no consequence. This has the additional benefit that polarity protection for the LED is not necessary. To make sure that the telephone line is not loaded unnecessarily, the LED is a high efficiency type. This type lights at a current as low as 2 mA, and this is, therefore the current arranged through it by resistor R1.

WARNING.

In spite of the liberal age we live in, it is highly probable that in many countries it is not allowed to connect the indicator across the telephone lines. Seek advice of your local telephone company that owns or operates the telephone network.

Read More

Audio Clipping Indicator

Detects clipping in preamp stages, mixers, amplifiers etc., Single LED display - 9V Battery supply unit

This circuit was intended to be used as a separate, portable unit, to signal by means of a LED when the output wave form of a particular audio stage is "clipping" i.e. is reaching the onset of its maximum permitted peak-to-peak voltage value before an overload is occurring. This will help the operator in preventing severe, audible distortion to be generated through the audio equipment chain. 

This unit is particularly useful in signaling overload of the input stages in mixers, PA or musical instruments amplification chains, but is also suited to power amplifiers. A careful setting of Trimmer R5 will allow triggering of the LED with a wide range of peak-to-peak input voltages, in order to suit different requirements. Unfortunately, an oscilloscope and a sine wave frequency generator are required to accurately setup this circuit. Obviously, the unit can be embedded into an existing mixer, preamp or power amplifier, and powered by the internal supply rails in the 9 - 30V range. The power supply can also be obtained from higher voltage rails provided suitable R/C cells are inserted. SW1 and B1 must obviously be omitted.

Audio Clipping Indicator Circuit Diagram

Parts:
R1___________1M 1/4W Resistor (See Notes)
R2,R3,R8_____100K 1/4W Resistors
R4,R6________10K 1/4W Resistors
R5___________5K 1/2W Trimmer Cermet or Carbon
R7___________2K2 1/4W Resistor
R9___________22K 1/4W Resistor
R10__________1K 1/4W Resistor (See Notes)
C1,C4________220nF 63V Polyester Capacitors
C2___________4p7 63V Ceramic Capacitor (See Notes)
C3___________220µF 25V Electrolytic Capacitor
C5___________10µF 25V Electrolytic Capacitor (See Notes)
D1,D2________1N4148 75V 150mA Diodes
D3___________LED (Any dimension, shape and color)
Q1___________BC547 45V 100mA NPN Transistor
IC1___________TL062 Dual Low current BIFET Op-Amp (or TL072, TL082)
SW1__________SPST Toggle or Slide Switch (See Text)
B1____________9V PP3 Battery (See Text)

Circuit operation:

The heart of the circuit is a window comparator formed by two op-amps packaged into IC1. This technique allows to detect precisely and symmetrically either the positive or negative peak value reached by the monitored signal. The op-amps outputs are mixed by D1 and D2, smoothed by C4, R7 and R8, and feed the LED driver Q1 with a positive pulse. C5 adds a small output delay in order to allow detection of very short peaks.

Notes:

• With the values shown, the circuit can be easily set up to detect sine wave clipping from less than 1V to 30V peak-to-peak (i.e. 15W into 8 Ohms). If you need to detect higher output peak-to-peak voltages, R1 value must be raised. On the contrary, if the circuit will be used to detect only very low peak-to-peak voltages, it is convenient to lower R1 value to, say, 220K omitting C2. In this way, the adjustment of R5 will be made easier.
• Using a TL062 chip at 9V supply, stand-by current drawing is about 1.5mA and less than 10mA when the LED illuminates. With TL072 or TL082 chips, current drawing is about 4.5mA and 13mA respectively.
• When using power supplies higher than 12V, the value of R10 must be raised accordingly.
• When using power supplies higher than 25V, the working voltage value of C5 must be raised to 35 or 50V.

Read More

110 and 220V AC LED Voltage Indicator

110 and 220V AC LED Voltage Indicator Circuit Diagram. Useful for power lines control, Simple, transformer less circuitry. This circuit, designed on request, has proven to be useful to indicate when the voltage in a power supply line is changing from 120V to 240Vac. It can be used in different circumstances and circuits, mainly when an increase in ac or dc supply voltage needs to be detected. D3 illuminates when the line voltage is approaching 120V and will remain in the on state also at 240V supply. On the other hand, D6 will illuminate only when the line voltage is about 240V and will stay on because the latching action of Q1, Q2 and related components. C1, D1 and D2 provide a low dc voltage in the 4.5V - 6V range in order to allow proper operation of latch circuit and LEDs.

Circuit diagram:

110 and 220V AC LED Voltage Indicator Circuit Diagram

Parts:
R1__________470R 1/2W Resistor
R2__________220K 1/4W Resistor
R3,R7_______470R 1/4W Resistors
R4__________1K 1/4W Resistor
R5__________2K2 1/4W Resistor
R6_________330R 1/4W Resistor
C1_________330nF 630V Polyester Capacitor
C2_________10µF 25V Electrolytic Capacitor
D1,D2______N4007 1000V 1A Diode
D3,D6______LEDs (Color and shape at will)
D4_________BZX79C10 10V 500mW Zener Diode (See Notes)
D5_________1N4148 75V 150mA Diode
Q1_________BC547 45V 100mA NPN Transistor
Q2_________BC557 45V 100mA PNP Transistor

Notes:

• D4 value could require some adjustment in order to allow precise switching of the circuit at the chosen voltage. If the case, please try values in the 8.2V - 15V range.
• Warning! The circuit is connected to 240Vac mains, then some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.

Read More

Clipping Indicator For Audio Amplifiers

A clipping indicator is a useful accessory on any audio amplifier. It indicates when the amplifier has reached its limit and is clipping the peaks of the audio signal. In practice, quite a lot of clipping can occur before you can hear it. So why is it necessary to know when an amplifier is clipping if you can't notice it? The answer is that clipping "squares up" the waveform and square waves contain lots of higher-frequency harmonics which can easily damage the tweeters in loudspeaker systems. This circuit is a true clipping indicator as opposed to the level indicators that are commonly used in preamplifier stages.

The problem with level indicators is that an amplifier's maximum output power is not constant. That's because the amplifier's supply rails are not regulated and so the maximum power available at any given instant varies, depending on the applied signal. The circuit is quite simple and is based on two BD140 PNP transistors and zener diode ZD1. During normal operation, Q1 is turned on via ZD1 and R1. As a result, Q2 is held off (since its base is pulled high) and so LED1 is also off. However, if the output signal subsequently rises to within 4.7V of the positive supply rail, Q1 turns off since it no longer has any forward bias on its base.

Clipping Indicator Circuit Diagram

As a result Q2's base is now pulled low via R2 and so Q2 turns on and lights LED1. (Note: the 0.6V drop across Q1's base/emitter is ignored here because ZD1 conducts before its rated voltage due to the very low current involved). Why choose 4.7V below the power rail as the turn-on point? The reason is that, due to the drive limitations and the nature of emitter followers, they can be expected to have at least 4V across them when they saturate (ie, clip). ZD1 can be increased to a 5V or 6.2V type if the circuit is to be used with a monster amplifier.

The value of R3 should be customized according to the amplifier's supply rail, so that LED1 operates with the correct brightness. To do that, first measure the amplifier's positive supply voltage, then use Ohms Law (R = V/I) to calculate the value of R3 for a current of about 20mA. As it stands, this circuit can only be used to monitor the positive-going half-cycles of the audio waveform. If you want to monitor the negative half-cycles as well, you will have to build a second circuit with the following changes: (1) reverse both LED1 and ZD1; and (2) use BD139 (NPN) transistors for Q1 & Q2. Note that, in both cases, you should use the earth inside the amplifier, as the speaker negative may not be earth (such as in a bridged output).

Author: Philip Chugg - Copyright: Silicon Chip Electronics Magazine

Read More

Water-Level Indicator Schematic

Here’s a simple water-level indicator for overhead tanks that uses three LEDs (LED1, LED2, and LED3) to indicate minimum, middle, and maximum water levels in the tank.

The sensor probes comprise A, B, C, and D, where A is the common probe and B, C, and D are meant for sensing the minimum, middle, and maximum levels, respectively. When water in the tank touches sensor wires A and B both, a small current passes from A to B through water and to the base of transistor T1 via resistor R1. As a result, transistor T1 conducts, causing LED1 to glow. Similarly, when water touches sensor C, LED2 glows to indicate that the water has reached the middle level. Finally, when water touches sensor D, LED3 glows to indicate the maximum level of water. Thus all the three LEDs glow when the tank is full. At this stage, the motor should be switched off manually.

Water-Level Indicator Schematic

The circuit can be easily assembled on a general-purpose PCB and enclosed in a wooden box. The three LEDs should be mounted on the front panel of the box with a spacing of about 4 cm between them. Short lengths of four 18 SWG copper wires may be used for sensor probes. For the common sensor A, a bare copper wire of 18 SWG should be used. For sensors B, C, and D three single-core PVC wires should be used, with their insulation removed to a length of one centimetre towards the ends. All the four wires may be tied around a 12.5mm dia. PVC tube with nylon thread at different heights, without touching each other (not shown in figure).

The sensor probes should be kept in the tank vertically and connected to the main circuit using four flexible PVC wires of different colours.

The circuit is powered by a battery eliminator or a 6V battery and kept near the motor switchboard. The current drawn by the circuit, when all the LEDs glow, is up to 50 mA, which is less than the current drawn by a 6V bed-lamp.

The circuit costs around Rs 50.

Sourced y: EFY Author:  P. Venkata Ratnam

Read More