Low Cost Deaf People Doorbell

This circuit provides a delayed visual indication when a door bell switch is pressed. In addition, a DPDT switch can be moved from within the house which will light a lamp in the door bell switch. The lamp can illuminate the words "Please Wait" for anyone with walking difficulties.

Circuit diagram

Notes:
The circuit uses standard 2 wire doorbell cable or loudspeaker wire. In parallel with the doorbell switch, S1, is a 1N4001 diode and a 12 volt 60mA bulb.

The bulb is optional, it may be useful for anyone who is slow to answer the door, all you need to do is flick a switch inside the house, and the bulb will illuminate a label saying Please Wait inside the doorbell switch or close to it.

The double pole double throw switch sends the doorbell supply to the lamp, the 22 ohm resistor is there to reduce current flow, should the doorbell switch, S1 be pressed while the lamp is on.

The resistor needs to be rated 10 watts, the 0.5 Amp fuse protects against short circuits.

When S2 is in the up position (shown as brown contacts), this will illuminate the remote doorbell lamp. When down, (blue contacts) this is the normal position and will illuminate the lamp inside the house. Switch S1 will then charge the 47u capacitor and operate the transistor which lights the lamp.

As a door bell switch is only pressed momentarily, then the charge on the capacitor decays slowly, resulting in the lamp being left on for several seconds. If a longer period is needed then the capacitor may be increased in value.

author: Andy Collinson
e-mail: anc@mitedu.freeserve.co.uk
web site: http://www.zen22142.zen.co.uk

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Digital Counter with an Interval of Kilometers

The circuit has been designed for a person who loves to jog or walk while measuring the distance that have been covered during the activity.

• 4093 – a quad 2-input NAND with Schmitt trigger inputs integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption.
• 4026 – a decade counter where the count advances as the clock input becomes high and has a maximum current of about 1 mA with a 4.5 V supply and 4 mA with a 9 V supply, which can light the appropriate segments of a common cathode 7-segment display.
• 4024 – a ripple counter with glitches that may occur in any logic state systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse; the count advances as the clock input becomes low on the falling edge as indicated by the bar over the clock label that is the usual behavior of the ripple counters which means a counter output can directly drive the clock input of the next counter in a chain.

The whole circuit may be placed in a small box and be placed in pants’ pocket where the 7-segment digital display shows the most significant digit D2 in the leftmost portion where it shows the 0 to 9 Km digits. The dot in between is always ON to segregate KM form hm. The least significant digit D1 is displayed at the rightmost part where it illustrates hundreds of meters and the dot is illuminating after every 50 meters of walking. In every two steps, a beeper will signal each count of unit, although it is not included in this circuit.

Circuit diagram :

Digital Counter with an Interval of Kilometers Circuit Diagram

A length of 78 centimeters is the calculated measure of a normal step which causes the LED to illuminate after 64 steps to signal a 50 meter distance. For a mercury switch, the illumination occurs every 32 steps. After 128 steps, the display will indicate 100 meters and so on. The SPST push button switch P2 is pressed only upon request in case of low battery consumption. Both push button switches P1 & P2 should be pressed together to reset the circuit in order to prevent accidental reset of the counters. The circuit should be considered as an approximation and not as a precision meter because it is very difficult to obtain the correct position of the mercury switch SW1 in the box where the degree of slope is being set.

The excessive bouncing of mercury switch is provided with certain degree of tolerance from the monostable multivibrator consisting of IC1A & IC1B. IC2 therefore is divided by 64 as a clean square pulse enters. The LED dot segment of D1 is driven by Q2 for every 32 pulses counted by IC2. At each monostable count, an audio frequency square is generated by IC1C for a short time. Using SW2 will disable the beep while the piezo sounder is driven by Q1. The power of beeper sound can be adjusted by trimming the value of R6. SW3 can be omitted when the display is disabled resulting to negligible current consumption.

The digital step counter circuit is widely used by people as their motion monitor while walking or jogging and other most ideal exercise possible. Some designs may come with a digital clock and backlight for easy reading during running, and belt clips.

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Simple Optical Theremin Circuit Diagram

Normally, Theremin works by detecting hand proximity using capacitive coupling method. A Theremin circuit shown in the schematic diagram below use different method to control the pitch. The oscillator of this tone generator, both the volume and frequency  are controlled using LDRs, a light sensitive electronic component, so we can call this circuit an optical Theremin.  Look at the following schematic diagram.

 Simple Optical Theremin Circuit Diagram

LDR1 control the frequency of this Theremin, while LDR2 control the volume level. We can place the LDR in two boxes where we can use our hand to control the aperture of the box, allowing smooth control of light amount that expose the LDR. 

This light is expected to come from ambient light, entering the box through the hand controlled aperture. This optical Theremin assume a stable ambient light to produce smooth control. The output will be heard on a small loudspeaker, but will be in very low volume. You can just amplify this output with a standard audio power amplifier to get better loudness.

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Low Cost Digital Code Lock

Many digital code lock circuits have been published in this magazine. In those circuits a set of switches (conforming to code) are pressed one by one within the specified time to open the lock. In some other circuits, custom-built ICs are used and positive and negative logic pulses are keyed in sequence as per the code by two switches to open the lock.

Digital Code Lock Circuit diagram :

A low-cost digital code lock circuit is presented in this article. Here the keying-in code is rather unique. Six switches are to be pressed to open the lock, but only two switches at a time. Thus a total of three sets of switches have to be pressed in a particular sequence. (Of these three sets, one set is repeated.) The salient features of this circuit are:

1. Use of 16 switches, which suggests that there is a microprocessor in-side.

2. Elimination of power amplifier transistor to energise the relay.

3. Low cost and small PCB size.

An essential property of this electronic code lock is that it works in monostable mode, i.e. once triggered, the output becomes high and remains so for a period of time, governed by the timing components, before returing to the quiescent low state. In this circuit, timer IC 555 with 8 pins is used. The IC is inexpensive and easily available. Its pin 2 is the triggering input pin which, when held below 1/3 of the sup-ply voltage, drives the output to high state. The threshold pin 6, when held higher than 2/3 of the supply voltage, drives the output to low state. By applying a low-going pulse to the reset pin 4, the output at pin 3 can be brought to the quiescent low level. Thus the reset pin 4 should be held high for normal operation of the IC.

Three sets of switches SA-SC, S1-S8 and S3-S4 are pressed, in that order, to open the lock. On pressing the switches SA and SC simultaneously, capacitor C3 charges through the potential divider comprising resistors R3 and R4, and on releasing these two switches, capacitor C3 starts discharging through resistor R4. Capacitor C3 and resistor R4 are so selected that it takes about five seconds to fully discharge C3.

Depressing switches S1 and S8 in unison, within five seconds of releasing the switches SA and SC, pulls pin 2 to ground and IC 555 is triggered. The capacitor C1 starts charging through resistor R1. As a result, the output (pin 3) goes high for five seconds (i.e. the charging time T of the capacitor C1 to the threshold voltage, which is calculated by the relation T=1.1 R1 x C1  seconds). Within these five seconds, switches SA and SC are to be pressed momentarily once again, followed by the depression of last code-switch pair S3-S4.

These switches connect the relay to out-put pin 3 and the relay is energised. The contacts of the relay close and the solenoid pulls in the latch (forming part of a lock) and the lock opens. The remaining switches are connected between reset pin 4 and ground. If any one of these switches is pressed, the IC is re-set and the output goes to its quiescent low state. Possibilities of pressing these reset switches are more when a code breaker tries to open the lock.

LED D5 indicates the presence of power supply while resistor R5 is a cur-rent limiting resistor.

The given circuit can be recoded easily by rearranging connections to the switches as desired by the user.

Source : www.ecircuitslab.com

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