Bathroom Fan Controller Circuit Diagram

Many bathrooms are fitted with a fan to vent  excess humidity while someone is showering. This fan can be connected to the light  switch, but then it runs even if you only want  to brush your teeth. A better solution is to  equip the fan with a humidity sensor. A disadvantage of this approach is that by the time  the humidity sensor switches on the fan, the  room is already too humid. Consequently, we decided to build a circuit  that operates by sensing the temperature of  the hot water line to the shower. The fan runs  as soon as the water line becomes hot. It continues to run for a few minutes after the line  cools down, so that you have considerably  fewer problems with humidity in the bathroom without having the fan run for no reason.

Naturally, this is only possible if you can  fit a temperature sensor somewhere on the  hot water line and the line does not become  warm if hot water is used somewhere else. We use an LM335 as the temperature sensor.  It generates an output voltage of 10 mV per  Kelvin. The output voltage is 3.03 V at 30 °C,  3.13 V at 40 °C, 3.23 V at 50 °C, and so on.  We want to have the fan switch on at a temperature somewhere between 40 and 50 °C (approx.100–150 °F). To do this accurately,we first use the opamps in IC2 to improve  the control range. Otherwise we would have  an unstable circuit because the voltage differences at the output of IC1 are relatively  small. IC2a subtracts a voltage of exactly 3.0 V from  the output voltage of IC1.

 Bathroom Fan Controller Circuit Diagram

It uses Zener diode  D1 for this purpose, so this is not dependent on the value of the supply voltage. The  value of R2 must be selected according to  the actual supply voltage so that the current through D1 is approximately 5 mA. It is  600 Ω with a 6-V supply (560 Ω is also okay),  or 2400 Ω (2.2 kΩ) with a 15-V supply. If you  have to choose between two values, use the  lower value. IC2b amplifies the output voltage of IC2a  by a factor of 16 ((R7 + R8) ÷ R8). As a result,  the voltage at the output of IC2b is 0.48 V at  30 °C, 2.08 V at 40 °C (104 °F), and 3.68 V at  50 °C (122 °F). Comparator IC3a compares this  voltage to a reference voltage set by P1. Due  to variations resulting from the tolerances of  the resistor values, the setting of P1 is best  determined experimentally. A voltage of 2.5 V  on the wiper should be a good starting point  (in theory, this corresponds to 42.6 °C).

When  the water line is warm enough, the output of IC3 goes Low. R10  provides  hysteresis  at  the  output  of  IC3a by pulling the voltage on the wiper of  the setting potentiometer down a bit when  the output of IC3a goes Low. IC3b acts as an  inverter so that relay Re1 is energised via T1,  which causes the fan to start running. After  the water line cools down, the relay is de-energised and the fan stops. If this happens  too quickly, you can reduce the value of R11  (to 33 kΩ, for example). This increases the  hysteresis. The circuit does not draw much current, and  the supply voltage is noncritical. A charging  adapter from a discarded mobile phone can  thus be used to power the circuit. If the supply voltage drops slightly when the relay is  energised, this will not create any problem.  In this case the voltage on the wiper of P1 will  also drop slightly, which provides a bit more  hysteresis on IC3a.

Author : Heino Peters - Copyright : Elektor

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Call Bell with Welcome Indication Circuit Diagram

This is a simple call bell circuit that displays a welcome message when somebody presses the call bell switch momentarily. the alphanumeric display can be fitted near the call bell switch. the circuit is built around two 555 ICs (IC1 and IC2), seven KLA511 common-anode alphanumeric displays (DIS1 through DIS7) and a few discrete components. For easy understanding,  the entire circuit can be divided into  two sections: controller and display. the controller section is built around  IC1 and IC2, while the display section is built around alphanumeric displays (DIS1 through DIS7). 

As shown in the circuit, both IC1 and IC2 are wired as monostable  multivibrators having time periods of around 5 seconds and 2 minutes, respectively. You can change the time period of IC1 by changing the values of resistor R12 and capacitor  C3. Similarly, the time period of IC2  can be changed by changing the values of resistor R2 and capacitor C1. Alphanumeric displays DIS1 through DIS7 are wired such that they show ‘WELCOME’ when the output of IC2  goes high. the circuit is powered by a 6V battery. Else, you can use the 6V, 300mA power adaptor that is readily available in the market. the 6V battery or power adaptor provides regulated 6V required to operate the circuit. 

Call Bell with Welcome Indication Circuit Diagram

A 6V DC socket is used in the circuit to connect the output of the adaptor if you don’t use the battery. Working of the circuit is simple. First, power-on the circuit using switch S2. LED1 glows to indicate presence of power supply in the circuit. Now if you press call bell switch S1  momentarily, it triggers  both the timers (IC1 and  IC2) simultaneously. IC1 produces a high output at its pin 3 for about five seconds. transistor t2 conducts and piezobuzzer PZ1 sounds for about five seconds indicating that there is  somebody at the door. At the same time, IC2  too produces a high out-put at its pin 3 for about two minutes. transistor  t1 conducts to enable the alphanumeric displays. the word ‘WEL-COME’ is displayed  for about two minutes  as DIS1 through DIS7  ground via transistor T1.

If switch S1 is pressed again within these two minutes, piezobuzzer PZ1 again  sounds for five seconds and the display continues to show ‘WEL-COME’. Assemble the complete circuit on a general purpose PCB and house in a small cabinet with call bell switch S1 and LED1 mounted on the front panel. At the rear side of the cabinet, connect a DC socket for the adaptor. Install the complete unit (along with the display) at the entrance of your house. Connect the 6V battery or 6V adaptor for powering the circuit. Configure switch 2 (used to enable/disable the call bell) in a switch board at a suitable location inside your house. If you don’t use a battery, connect the power adaptor to the DC socket on the rear of the cabinet. Close switch S2 only when you want to activate the circuit with  battery. Otherwise, keep it open when the 6V adaptor is in use.

EFY note. 

1. To avoid any shorting  during rain, waterproof the entire circuit assembly including alphanumeric displays (installed at the entrance) by covering it properly.

2.  the complete kit for this circuit is available with EFY associates  kits’n’spares. 

Author : S.C. Dwivedi - Copyright : EFY

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Simple Wireless Baby Monitor Project

This wireless baby monitor circuit using FM is designed to operate at a frequency of  100MHzefy tested. Its range of transmission is more than 100 metres with a 75cm wire antenna. The circuit is fully transistorised and so even a beginner can easily assemble it on a veroboard or the PCB whose pattern is given below for convenience.

Simple Wireless Baby Monitor Project

Circuit diagram of the wireless baby monitor circuit is shown in Fig. 1. It is built around a condenser microphone (MIC1), transistors BC549 (T1 and T3) and BC557 (T2), along with a few other components.

T1 and T2 provide high-gain audio to the VHF oscillator wired around T3. BC549 can oscillate well in VHF range. Frequency-modulated (FM) signals are transmitted through antenna ANT.1.

By adjusting trimmer capacitor VC1, frequency can be set within 88MHz -108MHz band. Using a good FM radio (analogue type) receiver, transmitted signals can be heard. Do not use a mobile phone as receiver.

This unit with 9-volt battery can be kept near a baby’s bed. Any sound generated by the baby including a cry can be monitored wirelessly from another room. Readily available 1-micro Henry inductor can be used as L1. It can also be home brewed as shown in Fig. 2.

Construction and testing

A single side PCB pattern of the wireless baby monitor circuit is shown in Fig. 3 and its component layout in Fig. 4. After assembling the circuit on a PCB, enclose it in a plastic case and keep it near the baby’s bed.

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Simple Dog Whistle for Ronja Circuit Diagram

This is the Simple Dog Whistle for Ronja Circuit Diagram. Ronja is the author’s dog, a beagle-mongrel,  who seems increasingly often to need to be  called to heel either with a shout or with a  whistle. And so the idea came about for an  electronic dog whistle that could produce  two alternating high-frequency tones. A  design like this has several advantages over  conventional whistles or calling.

 

Circuit diagram :

Dog Whistle for Ronja Circuit Diagram

 

• You can continue to carry on a conversation with your friends without having to  stop to whistle or call to your dog.
• Using high frequencies means that  the whistle sound is barely audible to  (especially older) humans and so is less  annoying to other people than conventional whistles or calls. As is well known,  dogs have rather better hearing than  we do and can hear frequencies of up to  40 kHz.
• The two alternating pitches mean that the  dog can more easily distinguish it from  other whistles.

 

The dog whistle is constructed from two  standard 555 timer ICs (or a single 556 IC),  both wired as astable multivibrators. The  first 555 oscillates at around 1.5 Hz and modulates the frequency of the second, which thus  switches between two different frequencies  every 0.7 seconds or so. The output of the second 555 is connected to a piezo sounder. If the  volume from the sounder used is insufficient, a small transistor amplifier can be added  between it and the output of the second 555. The circuit draws current only when activated by pressing S1. An optional green  LED indicates that the circuit is functioning.  When S2 is pressed the output frequencies  are reduced, making them more audible to  human ears for test purposes.

 

R1, R2 and C1 set the frequency of astable  multivibrator IC1. Diode D1 ensures that the  output is a symmetrical squarewave, by making C1 charge only via R1 and discharge only  via R2. Turning to IC2, where there is no diode in the  circuit, capacitor C2 is charged via R3 and R4  and discharged only via R4. With C2 = 22nF  the 555 oscillates at about 10 kHz; with S2  pressed, and hence C3 in parallel with C2, this  falls to about 1.8 kHz. Changing C2 to 10 nF  results in an even higher frequency (about  22 kHz), which can only be heard by dogs  and certain other animals. Setting C2 to 15 nF gives an output frequency of about 15 kHz. IC1 modulates the frequency of IC2 via R5. The green LED D2 is connected to the output  of IC1 via a series resistor and thus flashes at  the modulation frequency. The output from the piezo sounder at 10 kHz  (C2 = 22 nF) should be loud enough to verify  by ear. If desired, a more efficient piezo horn  tweeter can be used instead.

Author : Stefan Hoffmann

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Novel Liquid-Level Sensor Circuit Diagram

Normally, the level of a liquid in a container is determined by sensing changes in the capacitance or resistance between a pair of electrodes that are immersed in the liquid. Generally speaking, this technique requires fairly complicated circuitry to protect the electrodes against electrolysis (and associated corrosion). In addition, in many cases the liquid must be conductive for the measurement principle to actually be usable. The circuit presented here shows that an alternative approach is possible.

Novel Liquid-Level Sensor Circuit Diagram

Here we utilise the fact that a PTC resistor warms up in pro-portion to the amount of current flowing through it, with the result that its resistance increases. If a PTC resistor is immersed in a liquid, the additional warmth is dissipated in the liquid and the resistance remains nearly constant.

 If the level of the liquid drops below the immersion depth of the resistor, the change in the resistance can be easily sensed by a subsequent comparator stage. The PTC resistor should be isolated from the fluid into which it is immersed, in order to prevent undesirable electrolytic processes from taking place. A further improvement in the characteristics of the circuit can be achieved by using a logic circuit such as a microcontroller to apply power to the circuit only at predefined times and then switch off the power after sampling the comparator output.

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