Time delay relay circuit

What is a time delay relay circuit?

In a time delay relay circuit, relay contacts are closed or open after the preset time interval. Whereas in a normal relay contacts are open/closed immediately when coil is energized. It prevents the damaging effect of voltage spikes or input surge current by delaying the relay on/off.

Schematic diagram

Time delay relay circuit

Components list

  • 12v Relay
  • Transistor TIP122
  • D1 – IN4007
  • D2 – Zener diode 3.3v
  • VR1 – 100K
  • R1,R2,R3 –  1K
  • R4 – 330 Ohm
  • C1 – 1000 μF 25 v
  • C2 – 100 μF 25 v
  • LED – 2 nos

How it works ?

The circuit is based on RC time delay and zener controlled switch. When the power to the circuit is switch on, the 1000 μF capacitor charges through 100 k VR. Once the charge through the capacitor reaches 3.3 v zener diode starts conducting. As the zener is connected to the base of transistor, it triggers the transistor and it is turned on. The relay connected to the transistor is energized. The delay in the relay can be controlled by the VR 100K.

1 Watt audio amplifier for Mobile device

About LM4890

1 Watt audio amplifier for mobile device has been designed using LM4890 power amplifier. LM4890 is primarily designed for mobile phones and other portable devices. It is capable of delivering 1 watt of continuous average power to an 8 ohm load from 5v power supply. It has a distortion of less than 1%.

LM4890 does not require output coupling capacitors and therefore ideally suited for mobile phones and other low voltage applications where minimal power consumption is primary requirement. Power output at 5v is 1 watt and at 3.3 v it is approximately 0.4 watt.

Circuit diagram

LM 4890 - 1 watt audio power amplifier

Parts required.

  • C1= 1 μf 16v
  • C2=0.39 μF 50v
  • C3=1 μf 16 v
  • C4=25 pf
  • R1, R2, R3 = 20 k
  • J1 an dJ2 = Jumper head vertical mount
  • Speaker – 8 Ω

For more circuits on audio amplifier click here

Plus/minus supply using LMC7660

LM 7660 is a CMOS voltage convertor capable of converting a positive voltage in the range of +1.5 v – 10 v to the corresponding negative voltage of -1.5v to -10v using very few external components. Below is the schematic diagram for the same.

We can use this get +/-5v from different sources like from 9v battery. Adding LM7805 converter to a 9v battery or 9v power supply will reduce it to 5v as shown below.


  • IC1 – LMC7660
  • IC2 – LM7805
  • C1,C2 – 10μF 16V
  • C3 – 470 μF 25V
  • C4 – 2.2μF 16V
  • Battery – 9v


Bicycle USB Charger

This circuit has been developed to add a USB charging port to a bicycle for charging a mobile phone. The input supply for the circuit is produced by a dynamo (6V, 3W dynamo) in the bicycle.

The circuit is based on LM2596-5.0. It is a 3A step down voltage regulator. LM 2596 are available in fixed output voltage of 3.3, 5, 12v and an adjustable output version. The LM2596 series operates at a switching frequency of 150 kHz, thus allowing smaller-sized filter components than what would be needed with traditional lower-frequency switching regulators. The IC provides all active functions for a step-down (buck) switching regulator, capable of driving a 3A load with excellent line and load regulation.

Circuit & Part list

  • C1        1000uf
  • C2        470 uf
  • D1-D2        1N5818
  • D5        1N5818
  • J        Dynamo 6v 3W
  • L        33 u
  • LED
  • R          1k
  • SW        1 on/off switch
  • IC        LM2596
  • USB-A Connector

How it Works?

The alternating voltage generated by the dynamo is converted to DC by a full-wave bridge rectifier comprising Schottky barrier rectifier diodes D1 through D4 and a filtering electrolytic capacitor (C1). The output of the bridge rectifier, which is charged to peak value of AC voltage (nearly 10V), is input to switching regulator LM2596-5.0 to provide a regulated 5V (DC) output, which is suitable for charging mobile devices using the USB connector.

The 5mm LED (LED1) in the circuit indicates output status. Feedback connection FB (pin 4 on IC1) is connected directly to output voltage at electrolytic capacitor C2. The 33μH inductor (L1) should be rated for a DC current of at least 1A. The dynamo output is connected to the circuit by switch S1.

Adjustable Buck regulator with Low Ripple voltage

The LM2576-Adjustable Buck regulator using LM2576- Adj series are monolithic IC that provides all active functions for a step-down(buck) switching regulator. The output voltage is adjustable from 1.23 v to 37 v and are capable of driving 3 am. Load with excellent line and load regulation

It requires minimum number of external components.

Below are 2 version of it.

Fig 1 – LM2576-Adjustable Buck regulator output of 5V

LM2576-Adjustable Buck regulator 1

Fig 2 – LM2576-Adjustable Buck regulator output of 1.2v to 35 v

LM2576-Adjustable Buck regulator 2

If you want to voltage regulator circuit using LM78XX IC click here.

Solar mobile charger circuit

Solar mobile charger circuit using LM7805


solar mobile charger circuit

C1 and C2 – 4700 µf

This circuit is self explanatory. It uses solar panel to convert light energy to electrical energy to charge your mobile. It uses 5v regulator IC 7805 to provide constant current of 5v. Resistor R2 limits charging current to safer level.

Note: if your panel is rated less that 1.5 A then R2 is not required.

Components required for this project are listed below:

  1. Solar panel ( 5v 5 watt)
  2. Diode 1N4007
  3. IC 7805
  4. C1,C2 – 4700µf
  5. RED LED
  6. R1 – 1K
  7. R2 – 47 ohm
  8. USB female socket


Solar power supply circuit

This solar power supply circuit project uses the 1.2v rechargeable battery and solar panel.

The solar panel charges the battery when sunlight is bright enough to produce a voltage above 1.9v. The regulator transistor is designed to limit the output voltage to 5v.

solar power supply

courtesy- talkingelectronics.com( This solar power circuit is taken from talkingelectronics.com

White LED on 1.5 v Supply

In this post you will find circuit on how we can light up white LED on 1.5 v supply. Heart of the circuit is the transformer winded on 2.5mm by 0.5 mm core using 0.095 mm wire.

2 White LED on 1.5 v Supply

The circuit below uses only 1.5v to light up 2 white LED.

 Components required:
  • T1 – BC338
  • R1 – 2.7 K
  • C1 – 10 n
  • D1,D2 – Super bright LED
  • Transformer – 60 T, 40 T
  • S1 – On/off Switch
  • Battery 1.5 V

White LED Flasher

Below circuit uses only 1.5v to flash a white LED.

Components required
  • T1 – BC338
  • T2 – 2N2222
  • R1 – 2.7 K
  • R2 – 47 R
  • R3 – 330 R – 1K
  • C1 – 10 n
  • C2 – 100
  • D2 – 1N4148
  • Battery 1.5 v

For more post on LED click here.