Heart Rate Monitor

Strictly speaking, this simple circuit shouldnt work! How could anyone expect an ordinary light dependent resistor photo cell to see through a fingertip in natural daylight and detect the change in blood flow as the heart pulsates? The secret is a high gain circuit, based on a dual op amp IC which can be either the low power LM358 or the JFET TL072. The LDR is connected in series across the 9V battery supply via a 100kO resistor (R1) and the minute signal caused by the blood pulsing under the skin is fed to the non-inverting (+) input, pin 3, of IC1a via a 0.µF capacitor.

Pin 3 is biased by a high impedance voltage divider consisting of two 3.3MO resistors. The feedback resistors to pin 2 set the gain to 11 times. The output of IC1a is fed via a 0.47µF capacitor and 220kO resistor to IC1b. This is configured as an inverting op amp with a gain of 45 so that the total circuit gain is about 500. The output of IC1b is used to drive an analog meter which may be a multimeter set to the 10V DC range or any panel meter in series with a resistor to limit the current to less than its full-scale deflection. The prototype used an old VU meter with a 47kO resistor fitted in series.

Circuit diagram:

Heart Rate Monitor Circuit Diagram

Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. A light source such as a high brightness LED is not required. All that is needed is a reasonably well-lit room, preferably natural daylight, to produce a healthy swing of the needle. Only when the hands are very cold does it make it a little more difficult to accurately count the pulses. To check your heart rate, carefully position your thumb or finger over the LDR and count the meter fluctuations for a period of 15 seconds. Then multiply the result by four to obtain your pulse rate. The circuit can not be used if you are walking or running, etc.

 

 

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Monitor Life Extender

This circuit was designed to protect a computer monitor from overheating. It is recommended to attach this circuit to power users’ monitors! Most computer monitors of the CRT type fail owing to over-heating. After one or two hours of use, the rear of a monitor may become as hot as 45 degrees C, or 20 degrees above ambient temperature. Most heat comes from the VGA gun drivers, the horizontal circuit, vertical circuit and power supply. The best possible way to extract heat and so prolong monitor life (and save the environment) is to add a brushless fan, which is lighter, energy-wiser and more efficient than a normal fan.

In the diagram, diodes D2, D3 and D4 sense the monitor’s temperature. These diodes have a total negative temperature coefficient of 6 mV per degree Celsius. To eliminate noise, shielded wire should be used for the connection of the temperature sensor to the circuit sensor. The +12-V supply voltage is borrowed from the computer’s power supply. Alternatively, a mains adapter with an output of 12 VDC may be used. C1 and C2 are decoupling capacitors to eliminate the ripple developed by switching or oscillation. R1 provides bias current to D1, a 6-V zener diode acting as a reference on the non-inverting pin of opamp IC2.B.

IC1, a ‘precision shunt regulator’ raises the sensor diodes’ voltage to just over 6 V depending on the adjustment of P1. C4 is the decoupling capacitor with the sensor network. Integrator network R4-C5 provides a delay of about 3 seconds, transforming the on/off output signal of IC2.B into an exponentially decreasing or increasing voltage. This voltage is fed to pin 3 of the second opamp, IC2.A. The hard on/off technique would produce a good amount of noise whenever the load is switched, hence an alternative had to be found. IC3, a TLC555, is used as an astable multivibrator with R5 and C6 controlling the charging network that creates a sawtooth voltage with a frequency of about 170 Hz.

Circuit diagram:

Monitor Life Extender Circuit Diagram

This sawtooth is coupled to pin 2 of IC2.A, which compares the two voltages at its input pins and produces a PWM (pulsewidth modulated) output voltage. The sawtooth wave is essential to the PWM signal fed to power output driver T1 by way of stopper resistor R6. The power FET will switch the fan on and off fan according to the PWM drive signal. The back emf pulses that occur when T1 switches on and off are clamped by a high-speed diode, D7. Initially, turn P1 to maximum resistance.

Blow hot air from a hair dryer onto the sensor-diodes for a minute or so, then get the temperature meter near the sensor diodes and adjust P1 slowly towards the minimum resistance position with a digital meter hooked up on pin 7 of IC2.B. Roughly calibrate the temperature to 40 degrees C. At this temperature, the meter will show approximately 12 V. The circuit will draw about 120 mA from its 12-V supply.

Author: Myo Min - Copyright: Elektor July-August 2004

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AC Power Monitor

This AC Power Monitor continuously watches the AC power line voltage for both under-voltage and missing cycles. When it detects a total of 5 or 6 consecutive missing half-cycles (50mS, 50/60HZ), it drops a relay and starts a timer. A power contactor is slaved to the relay contacts. Its purpose is to protect power-loss sensitive industrial equipment from brief power glitches by preventing immediate restart.


After the specified time delay (2 to 10S), the equipment may automatically restart. Although this may seem like a simple task, note that the condition sensed is the absence of a periodic signal and that the timer may or may not have external power available. electroschematics.com is not really into industrial controls, but this may be the beginning—this is where I spent most of my life.
This function could also be performed by a PLC (programmable logic controller) with a UPS (unterruptable power source), but such would be expensive and consume additional panel space.

Power Monitor Schematic

Two key components (transformer and relay) are documented on the power monitor schematic — there is no bill of materials.
Power supply
Key to this functioning properly is its power-loss ride-through capability. Note that this circuit continues to time out even after the power is interrupted. To run the circuitry for the entire time period, energy is stored in a large filter capacitor (10,000uf). That way if the power resumes during the timeout period, the timer remains alive and functioning so that the sensitive equipment may not restart.
The raw capacitor voltage ranges from about 14 to 24V, and is series regulated to 12V via U2, an LM7812 voltage regulator.
The power transformer (T1) provides low voltage power and power line isolation. Its primary is reconnectable for 115 /230VAC.
Capacitor Bleeder
At the end of the timeout period, the bleeder circuit kicks in to dump the remaining charge. A 4000 series CMOS latch consisting of U3B & U3C is set by the positive transition of U1-3. Should the power resume while the capacitor is bleeding down, the latch is reset via the signal at the collector of Q3 so that the bleeder driver Darlington (Q1 & Q2) turns off. When all of the charge is dissipated by the bleeder resistor (R3), the circuit is free to immediately restart and again monitor voltage glitches.
The MC14093 is a good choice for the power monitor application. It has Schmitt trigger inputs for handling slow input signal transitions, and may be powered by the 12V Vcc. It is identical in pin-out to the more common CD4011. Note that the CD4011 should also function acceptably, but I did not try it in the circuit.
Timer
The timer is the good old 555 (actually, it is the TLC555 due to its low quiescent current). It is configured as a monostable multivibrator that is triggered via a positive voltage at U1-6. To assure that the internal latch of the 555 is set in the proper state during power-up, C7 works against Vcc (rather than common) and U1-2 is held low for 0.5 sec via U3A, C6 and R10.
The timeout period may be increased substantially by increasing the size of the timing capacitor C7. Note that C1 must be adjusted likewise to provide additional ride-through. 60 seconds is not unreasonable.
AC voltage threshold detector
Note that the cathodes of D1 & D2 are unfiltered—this is necessary for rapid sensing of power loss—a filter capacitor would tend to hold up the voltage thus interfering with the voltage loss timer. The filter capacitor (C1) is isolated via D3. The Vbe of Q3 (0.65V) is the voltage threshold comparator. This voltage is multiplied by the ratio of (R4, R5 & R6) to R6.
The function of the voltage threshold comparator is to keep C5 discharged. At each line voltage zero-crossing C5 charges slightly, but requires a full 50mS to reach the threshold of U1-6. Should the voltage remain below the threshold of Q3, C5 starts to charge significantly—if 5 or 6 half-cycles are lost in succession, the voltage across C5 charges to the 8V threshold of U1-6 and triggers the timer.
Proper adjustment of R4 is required. To do so, connect a Variac to the power terminals and reduce the voltage to the desired AC threshold voltage (e.g. 170VAC). Then increase the value of R16 until the LEDs change state.
Power Monitor Oscillographs

Power Monitor Photos

Glossary of undocumented words and idioms (for our ESL friends)
e.g. common abbreviation – example given

TLC555 datasheet

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Heart Rate Monitor

Strictly speaking, this simple circuit shouldnt work! How could anyone expect an ordinary light dependent resistor photo cell to see through a fingertip in natural daylight and detect the change in blood flow as the heart pulsates? The secret is a high gain circuit, based on a dual op amp IC which can be either the low power LM358 or the JFET TL072. The LDR is connected in series across the 9V battery supply via a 100kO resistor (R1) and the minute signal caused by the blood pulsing under the skin is fed to the non-inverting (+) input, pin 3, of IC1a via a 0.µF capacitor.

Pin 3 is biased by a high impedance voltage divider consisting of two 3.3MO resistors. The feedback resistors to pin 2 set the gain to 11 times. The output of IC1a is fed via a 0.47µF capacitor and 220kO resistor to IC1b. This is configured as an inverting op amp with a gain of 45 so that the total circuit gain is about 500. The output of IC1b is used to drive an analog meter which may be a multimeter set to the 10V DC range or any panel meter in series with a resistor to limit the current to less than its full-scale deflection. The prototype used an old VU meter with a 47kO resistor fitted in series.

Circuit diagram:

Heart Rate Monitor Circuit Diagram

Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. A light source such as a high brightness LED is not required. All that is needed is a reasonably well-lit room, preferably natural daylight, to produce a healthy swing of the needle. Only when the hands are very cold does it make it a little more difficult to accurately count the pulses. To check your heart rate, carefully position your thumb or finger over the LDR and count the meter fluctuations for a period of 15 seconds. Then multiply the result by four to obtain your pulse rate. The circuit can not be used if you are walking or running, etc.

 

 

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Motorcycle Battery Monitor Circuit Diagram

A circuit for monitoring the status of the battery and generator is undoubtedly a good idea for motorcyclists, as for other motorists. However, not every biker is willing to drill the necessary holes in the cockpit for the usual LED lamps, or to screw on an analogue accessory instrument. The circuit shown here manages to do its job with a single 5-mm LED, which can indicate a total of six different conditions of the onboard electrical system. This is done using a dual LED that can be operated in pulsed or continuous mode (even in daylight). Built on a small piece of prototyping board and fitted in a mini-enclosure, the complete circuit can be tucked inside the headlamp housing or hidden underneath the tank.

 

The heart of the circuit is IC2, a dual comparator. The comparator circuit is built without using any feedback resistors, with the indication being stabilised by capacitors C4 and C5 instead of hysteresis. Small 10-µF tantalum capacitors work well here; 220-µF ‘standard’ electrolytic capacitors are only necessary with poorly regulated generators. Voltage regulator IC1 provides the reference voltage for IC2 via voltage divider R2/R3. The onboard voltage is compared with the reference voltage via voltage dividers R4 /R5 and R6/R7, which are connected to the inverting and non-inverting comparator sections, respectively.

Using separate dividers allows the threshold levels to be easily modified by adjusting the values of the lower resistors. IC2a drives the anode of the red diode of LED D4 via pull-up resistor R10. The anode of the green diode is driven by IC2b and R11. T2 pulls R11 to ground, thereby diverting the operating current of the green diode of the LED, if the voltage of the electrical system exceeds a threshold level of 15 V (provided by Zener diode D3). The paralleled gate outputs on pins 10 and 11 of IC3 perform a similar task. However, these gates have internal current limiting, so they can only divert a portion of the current from the red diode of the LED.

The amount of current diverted depends on the battery voltage. The two gates are driven by an oscillator built around IC3a, which is enabled via voltage divider R14/R15 and transistor T1 when the battery voltage is sufficiently high. Depending on the state of IC3a, the red diode of the LED blinks or pulses. The circuit is connected to the electrical system via fuse F1 and a low-pass filter formed by L1 and C1. If you cannot obtain a low-resistance choke, a 1-Ω resistor can be used instead. In this case, the values of C3, C4 and C5 should be increased some-what, in order to help stabilise the indication. D1 protects the circuit against negative voltage spikes, as well as offering protection against reverse-polarity connection. Due to its low current consumption (less than 30 mA), the circuit could be connected directly to the battery, but it is better to power it from the switched positive voltage.

Source : www.extremecircuits.net

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