Time Delay Circuit

Looking for a simple time delay circuit? In the design of analog circuits, there are times when you would need to delay a pulse that came into a circuit before being used for the next process. This circuit uses a 555 timer to delay a pulse that comes in to a maximum time of 75 seconds. The timing of the delay can also be changed by changing the resistor value of VR1 and the capacitor value of E based on the time delay formula of t=0.69RC.




In order for the output to go high, the reset pin of 555 timer (pin 4) must be high and the TRIGGER pin (pin 2) voltage level must be below a third of the level of the power supply to the IC. When there is no pulse being applied to the input, transistor Q1 will turn ON and capacitor E is charged.


Once a pulse is applied to the input, transistor Q1 will turn OFF and pin 4 reset pin is held to high. This caused the capacitor E1 to be discharged through VR1 resistor. The time delay will depend on the discharged of capacitor E to a third of the supply before the output of 555 goes high. Experiment with different values of VR1 and E to get different time delay.
If the maximum value of potentiometer is set to 5M ohm, the time delay of the pulse will be 75 seconds.

Parts List


This is a good project to embark on if you are a beginners to electronic design. There are only 5 components needed to construct this circuit. It will provide you a good hands-on experience of using the 555 integrated circuit, one of the most commonly available IC widely in use for the past decades.

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Automotive Battery Charger Project

This automotive battery charger project is based on National Semiconductor LM350 3A Adjustable Regulator. It is designed to charge 12V lead acid batteries. When the Switch SW1 is pushed, the output of the charger will go up to 14.5 V. The initial charging current is limited to 2A.


As the charge of the battery continues to rise, the charging current decreases to 150mA and the output voltage is reduced to 12.5V. At this stage, the charging is terminated and the light emitting diode lights up to indicate that charging process is completed.

http://www.electronics-project-design.com/Timer.html

     CK1619 - DUAL HI/LO SWITCHED RELAY BOARD
There are many applications where you need quick
availability of a relay for connection to a particular piece
of apparatus. We found in developing our kits that we
always needed to have a relay mounted on a PCB within
arms reach. So we thought if we need it then many others
probably do too.
But there are problems. Relay pinouts and packages are
not standardized. And then what voltage of relay should
we use - 3V, 6V, 9V, 12V or 24V. So we chose a
commonly available miniature relay, and we have
supplied a 12V version of it here.
We have used two Goodsky RWH-SH-112D 12V relays,
coil resistance 400 ohm. It is rated to switch 250VAC at
12A. However, because of the PCB track thickness and
width we do not recommend using the relay to switch
more than 5A. If you want to switch more current then we
suggest you solder some heavy duty wire links, on the
bottom of the PCB, from the relay contacts to the screw
terminal block (effectively in parallel with the PCB
track).
The terminals blocks are rated 300V / 10A.
Assembly. Follow the overlay. Solder the resistors first.
Make sure to get the diode and the IC around the correct
way.
How it Works. The kit is based around the ULN2003A
IC, a 7-channel high voltage, high current relay driver.
The inputs are TTL compatible, allowing them to be
directly connected to logic circuits operating from a
supply voltage of 5V.
Each driver is effectively a logic inverter with an open
collector output, meaning the ‘load’ is connected between
the output pin and V+.
Looking at the schematic we see that there are two
identical circuits, one for each relay. Operation is the
same for both so we will refer to the RL1 circuit only in
the following explanation.
There are two inputs that can be used to operate the relay,
marked LO and HI. As the names suggest a low on the
LO input will operate the relay. Similarly a high on the HI
input will also operate the relay.
The LO input operates the relay via IC1:A and IC1:B. A
low level input will be inverted by IC1:A and its output
will be high. Then this high is inverted again by IC1:B to
give a low output to operate the relay. Resistor R1 holds
the input high when not used.
Now ‘hang on’ you might say – why invert a low to a
high then just invert it low again? Why not connect the
LO input direct to the relay and forget about using IC1:A
and IC1:B? Good question. The answer is that if you
connect the LO direct to the relay you lose all control
about what voltage the LO input can be. An input of 3V
for example, will trigger the relay closed. This will
probably be quite undesired. By using the two relay
drivers to process the signal the LO must be no more than
0.8V. Anything over that will not trip the relay and you
have full control.
The relay can also be operated via the single inverter
IC1:C. In this case a high level on the HI input is inverted
by IC1:C and the resulting low output will operate the
relay. Resistor R3 holds the input low when it is not used.
So, the relay will be operated when either the LO input is
low (0 – 0.8V) or the HI input is high (2.4V – 12V.).
Note that the outputs of IC1:B and IC1:C are connected
together. At first glance it might seem that they would
destroy each other if one was high and the other low. This
cannot happen because the outputs are ‘open collector’,
meaning that the inverter can drive the output low but it
relies on an external device to pull the output high. In this
case the external device is the relay.
Tying open collector outputs together like this is known
as a “wire OR’ configuration. It means that the relay is
operated when either the IC1:B output OR IC1:C output
is low.
Diode D1 provides reverse polarity protection in case the
power supply to the kit is connected the wrong way
around.
Specifications.
Operating voltage: 12V DC
Input low voltage: 0 – 0.8V
Input high voltage: 2.4 – 12V
COMPONENTS
10K resistor 5% 1/4W brown black orange 6
1N4004 diode 1
3 pole terminal block 4
ULN2003A IC 1
16 pin IC socket 1
RWH-SH-112D 12V relay 2
K156 PCB 1
  **   See out website at http://www.ElectronicKits.com/
CK1619 - DUAL HI/LO SWITCHED RELAY BOARD