DELAY TIMER PROJECT-BASIC ELECTRONICS

Introduction to Delay Timer
In this Delay Timer project, all analog parts are being used with the thyristor as a device that switches an AC Relay ON or OFF depending on the timing of the RC circuit. The input mains supply used ranges from 220VAC to 240VAC and an AC relay (220-240VAC) is used to switch a load. The load to be switched must be within the current and relay ratings. This circuit is useful for use of devices that need to be OFF for a minimum of 150 - 210 secs after a mains supply have cuts off. Devices such as compressors and halagon lamp cannot be OFF and ON repeatedly within a short period of time as it will cause damage to the devices.
The use of microcontroller based devices are not reliable in that if the power supply cuts off and came back again in a short period of time, it will reset and "forgotten" its previous state. The use of RC circuitry as a timer circuit is reliable and is not susceptible to "memory loss" as in the case of microcontroller.
If a microcontroller based solution is used, extra circuitry such as backup battery or supercapacitor need to be incorporated in order to retain the memory of the MCU and to ensure that the clock still runs even after the supply has cuts off.
This project should be handled by experienced electronics designer as its part are powered on directly from the mains supply. As all parts is "live", one may get electric shock if care is not taken when testing the circuit. Some parts may "burst" if there are some short circuit in the circuit. It is not recommended to use breadboard to test the circuit. Circuit should be tested using printed circuit board and an isolating variable transformer where the voltage is slowly ramped up from zero.
Once tested working, the components should be potted using epoxy with only the terminals exposed. All parts are potted to prevent users from touching the parts.



google_protectAndRun("ads_core.google_render_ad", google_handleError, google_rend
Schematic Diagram
The schematic below shows the circuit diagram of the ON delay timer. Once the mains power supply cuts off, the relay will only be able to turn ON after a period of 150 - 210 secs depending on the tolerance of the RC circuit represented by resistor R7(5.1 Mohm) and electrolytic capacitor E2(47uF). More accurate timing can be achieved by using low tolerance resistor and capacitor.
The thyristor used can be either 2P6M or MCR106-8 or equivalent parts available in the market. Relays used should have coil ratings below 1A in order not to overheat the SCR. No heatsink is required for the SCR.
At power on, there is no charge at E2, hence the transistor Q2 will be forward bias and turn ON when Q3 turn ON. Once these two transitors are ON, the SCR will turn ON as well. The use of C1 and R6 across the SCR acts as a snubber circuit to reduce the switching noise generated by the SCR when it turns OFF/ON. During the ON stage of the SCR, the capacitor E2 is charged to its maximum value. When the mains supply cuts off, the charge at capacitor E2 will cause the base of transistor Q2 to be reverse bias and cannot turn ON until almost all the charges have been discharged through resistor R7. Once the charge has been discharged (which will take around 150 - 210 secs for the values shown), transistor Q2 will be able to turn ON.
The timing of the circuit can be changed by reducing or increasing the RC values of R7 and E2.

Parts List The parts list of the delay timer circuit is as shown below.

STEREO SHUTOFF

It happens to just about everyone. One minute you’re listening to the hi-fi, the next you’re called away to answer the doorbell or a phone call. You forget all about the music, the record plays through, the automatic turntable shuts off — but the amplifier stays on until you happen to pass by and notice the glow from the pilot lamps. Yet, this simple circuit, which you can throw together in less than an hour, will automatically turn off the amplifier when the turntable shuts off. The relay coil voltage is taken from across the phonomotor; when the turntable motor is on, relay K1 closes and applies power to AC socket SO1; When the turntable shuts off, removing voltage from the motor, K1 opens, disconnecting power from outlet. Because the turntable automatic shutoff switch might not be able to carry the amplifier load, the AC power for SO1 is taken off before the automatic shutoff switch. Switch S1 bypasses the relay contacts and applies power to the socket even when the turntable is off.

Parts List For Stereo Shutoff

K1 – 117V AC relay with contacts rated at least 5 amperes at 117V AC (Radio Shack 275-207)

S1 – Switch, SPST (Shutoff bypass)

SO1 – AC Socket

HOW TO MAKE A HEARING AID?

[caption id="" align="aligncenter" width="254" caption="Image via Wikipedia"][/caption]

DESCRIPTION

Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components.

On moving power switch S to ‘on’ position, the condenser microphone detects the

sound signal, which is amplified by transistors T1 and T2. Now the amplified signal

passes through coupling capacitor C3 to the base of transistor T3. The signal is further

amplified by pnp transistor T4 to drive a low impedance earphone. Capacitors C4 and C5

are the power supply decoupling capacitors.

The circuit can be easily assembled on a small, general-purpose PCB or a Vero board. It operates off a 3V DC supply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the sensitivity of the condenser microphone house it inside a small tube.

This circuit costs around Rs 65.

DOWNLOAD THE CIRCUIT DIAGRAM

HOW TO MAKE A WIRELESS ELECTROCARDIOGRAM(ECG) MONITOR-BIOMEDICAL PROJECTS

[caption id="" align="aligncenter" width="300" caption="Image via Wikipedia"][/caption]

Introduction

The electrocardiogram (ECG or EKG) is a noninvasive test used to measure the electrical activity of the heart. An ECG can be used to measure the rate and regularity of heartbeats, the position of the chambers, the presence of any damage to the heart and the effects of drugs and devices used to regulate the heart. This procedure is very useful for monitoring people with heart disease or to provide diagnosis when someone has chest pains or palpitations.

Leads are placed on the body in several pre-determined locations, usually the extremities or the front of the chest, to provide information about heart conditions. For our final project, we implemented a wireless electrocardiogram monitor.

The following figure describes the overall high-level design for our ECG monitor:

Three leads are placed on the subject --usually one on each side of the chest and on the lower abdomen. This signal is sent to the amplifier where it is amplified by a factor of one thousand. The signal is then sent to the voltage to frequency converter (VFC), which converts the signal to a frequency so that it can be transmitted. Since we desired the amplifier, VFC and transmitter to operate using only a single 9 V battery, a separate source splitter circuit was used to provide the proper voltage to each of the components.

Once the signal is received using the radio receiver, a voltage summer is used to add an offset voltage of approximately 800 mV to the signal in order to make the signal entirely positive. This signal is then amplified by a factor of three so that its maximum value exceeds the threshold of the required voltage for the frequency to voltage converter (FVC). After the signal is passed through the FVC, the output signal is displayed on an oscilloscope.

Hardware

The following schematic shows the amplifier section of the circuit:

DOWNLOAD THE SCHEMATIC

With the reference lead of the the subject placed to ground, each of the input chest leads is sent to an input of the INA121 instrumentation amplifier. Using a 4.7k resistor, a gain of 11.7 results from this stage. Following the instrumentation amp, the signal is passed through two 10 uF capacitors, placed back to back. The capacitors are used to prevent baseline drift in the ECG signal. Putting two directional capacitors back-to-back forms a bi-directional capacitor. A time constant of 0.5 second was chosen to approximate the frequency of a standard ECG signal (a resistor of 100k can be connected to ground after the capacitor in order to make a time constant of 0.5 sec, but we found that this resistive element is unnecessary). This section is followed by two inverting amplifiers each with a gain of ten. The total gain of this part of the circuit is approximately equal to one thousand.

The following schematic, obtained from the LM231 data sheet, shows the VFC circuit used in our monitor:

DOWNLOAD SCHEMATIC1

V_logic is connected to the power supply Vs and all ground are connected to the negative terminal of the battery. Fout is a square wave of varying frequency with a maximum amplitude of Vs. A voltage divider is needed (200k and 10k variable resistor connected in series) is needed at pin 3 to scale the voltage down to around 20 -50 mV.

The output of the voltage divider is connected to the input of the transmitter shown in the circuit diagram below.

Please note: We added everything on this schematics except for the 22k and the offset adjust. We neglected those two elements completely.

The following schematic describes the the source-splitting and transmitter section of our project:

The source-splitting amplifier allows three different potential reference: + 4.5V, -4.5V and Ground. Since the BA1404 can only have +3V as its power supply, we used two diodes to create a total drop of 1.4V and this allows the transmitter to function properly. Another advantage to this setup is that every elements on this circuit can be powered off a signal 9V battery. Although not noted on this schematics, one should know that the input to the transmitter should be on the order of mV (5 - 50mV). Implementing a variable voltage divider to the input is very important. The nice thing about this setup is that one does not need an DC offset circuitry to adjust ECG signal from the output of the amplifier. Since our VFC is powered between -4.5V and +4.5V, it has a 4.5V offset already. If the ECG signal is centered at 0V with a swing from -0.5 to +0.5V, then the VFC sees it as 3.5V to 4.5V swing. It is important to know that VFC cannot have negative voltage as its input. Furthermore, making an inductor at the tunable FM transmitter range is a painstaking process. We found that by turning a wire 4 times around the pen allows the signal to be transmitted at 90 MHz.

Receiver Circuitry:

The following schematic shows the voltage summer and amplifier section of the project:

This section consists os a summing amplifier with a gain of one cascaded with an inverting amplifier with a gain of approximately two. The summer takes the input obtained from the receiver (a FM radio tuned at approximately 90 MHz) and adds it to a constant voltage obtained using a simple voltage divider. This signal is then sent to an inverting amplifier which provides a gain of two and inverts the signal after it has been inverted by the summing amplifier. The reason we amplified the signal is due to the fact that FVC (LM231) needs at least a 2V peak-to-peak amplitude. The signal coming from the radio receiver has a peak-to-peak amplitude around 500 mV. Increasing the volume will normally increase the signal amplitude but it will also decrease the signal-to-noise (SNR) ratio. The amplifier we designed at the receiving end increase the signal to at least 5V of peak-to-peak amplitude, which is sufficient for FVC conversion.

The following figure shows the FVC circuit, also obtained from the LM231 data sheet:

Equation used to calculated the voltage output due to the frequency response

Results

One of the most difficult parts about this project is setting up the VFC (Voltage-to-Frequency) and FVC (Frequency-to-Voltage). Through many experimentations, we found out that the VFC can not convert a signal varying more than 16 Hz into pulse train. For example, when we fed in a 100 Hz sine wave, we were getting a constant square as the output (thus a constant DC voltage). This could be a potential problem for ECG transmission since the QRS peak can occur as fast as 20 to 50 Hz. However, when we fed in square waves of varying frequency into the FVC, we could get a varying DC voltage as expected. This is probably the reason why we could not receive a nice-looking ECG waveform on the receiving end. Also, we believe that the signal was attenuated during the transmission process. We had a difficult time receiving a nice looking square wave from the FM radio receive. However, we were able to fix that problem by increasing the volume on the radio to create better rising and falling edges for the FVC. Finally, we noticed that our transmission range is about 10 feet, which is not very useful for a wireless ECG. The pictures below demonstrate our final result:

Transmitter Section (this includes, pre-amp, source-splitting, FM transmitter and VFC).

IMAGES OF CIRCUIT

1,

2,

3,

4,

5,

6,

REFERENCES

LM231.pdf (VFC and FVC)

LM158.pdf (Operational Amplifier)

INA121.pdf (Instrumentation Amplifier)

http://www.medicinenet.com/Electrocardiogram_ECG_or_EKG/article.htm

http://www.nlm.nih.gov/medlineplus/ency/article/003868.htm