Timer Based Cell Phone Charger Circuit

A simple cell phone charger with timer circuit is presented in the following article, which could be used for charging a given Mobile phone for a specified predetermined length of time. The idea was requested by Mr. Saad.

Circuit Objectives and Requirements

1. Could you design me this charger circuit ? Input 230V 60 Hz, and the output 3 USB port for charging Smartphones.
2. What I need in this circuit is a Timer (Three sets of time), 30 mins, 60 mins and 120 mins.
3. So I connect my phone to any of the three usb port and press (on/off) switch then the time start for example 60 mins then the power cut off.
4. Hope you understand my request.

Circuit Diagram

Circuit Operation

The proposed cellphone charger circuit with timer can be seen in the above figure, the design primarily comprises of an IC 4060 timer stage and a DC to DC multi cell phone charger stage.

The cell phone charger section is a standard LM338 based charger circuit, wherein the output is bifurcated into 5 individual charging outputs facilitating charging 5nos individual cellphones. From these outputs 3 channels could be utilized for the intended 3 cell phone charging, at a rate of 1500mAH each. The series resistors may be accordingly calculated using Ohms law, as given below

R = V/I = 5/1.5 =  3.33 ohms, 10 watts each

R2 in the LM338 circuit must be appropriately adjusted to achieve around 5V across the output terminals or across C2 terminals.

The timer stage is made up of the IC 4060 whose pinouts are also configured in its standard timer/counter mode.

P1 may be adjusted to get a delay time of around 120 minutes on pin#3, which would allow pin#2 to produce a delay of 60 minutes, and pin#1 a delay of 30 minutes.

Initially when power is applied across the indicated  mains input terminals, the cellphone charger circuit with timer does not respond and stays deactivated.

However, the moment the given push button is pressed, causes the N/O side of the relay to get connected with the other unconnected mains wire.

This momentarily connects the AC  mains with the transformer leads, which in turn powers the rectifier stage enabling a momentary DC supply input for the IC 4060 timer stage. 

This momentary supply to the IC 4060 stage activates the counting of the timer, and simultaneously produces an initial zero potential at the base of the relay driver BC557 transistor, switching ON the relay from N/C to N./O points.

As soon as this happens, the relay contacts now take-over the push-to-ON switch connections and allows the AC to flow through these contacts into the transformer primary.

This ensures that, now even if the push button is released, the circuit is able to get latched into the powered position enabling the LM338 to begin charging the attached cell phones and the timer IC 4060 to count the stipulated amount of time via the pot P1.

As soon as the counting of the IC 4060 gets elapsed, pin#3 (pin#1/2 whichever is selected) turns high, switching of the BC557 and the reverting relay contacts from N/O to N/C.

This action instantly switches OFF and disconnects the mains AC from the transformer, deactivating the whole process and bringing the whole system into its original standby position.

This cellphone charger timer circuit could be yet again initiated simply by pressing the push button for the next charging cycle.

Time Delay for the IC 4060 can be calculated using the formula:

f(osc) = 1 / 2.3 x Rt x Ct

where Rt = R2 +P1 (in Ohms)

Ct = C1 (in Farads)

Parts List

Resistors, All 1/4 watt 5%

2M2 = 1
22K - 1
10K = 1
1M = 1
120 ohms = 1
1M pot= 1
5K pot = 1

Capacitors
1uF/50V non-polar = 4
0.33uF = 1
470uF/25V = 1
1uF/25V electrolytic = 1

Diodes, 1N4007 = 5
Transistor, BC557 = 1
IC, LM338 = 1

Relay,12V/400 ohm = 1
Push button = 1

Transformer = 0-12V/5 amp

Output resistors as per the given formula