For the past couple of years, I have been plagued with dodgey modem operation. It would (for no apparent reason) just lock up. The connection is there but no data is being transfered. This in itself is easily solved, remove power for thirty seconds, re-apply power and your good to go. Then came WIFI. Same problem, almost same solution. Remove power to both the modem and the router, wait thirty seconds, re-apply power to the modem, wait for it to settle, then re-apply power to the router, sorted.

Now, the above method is fine and dandy if you have both the time and the know how, but what if your not around, or your grandma is getting the same problem. Sure you can teach them, but.., wouldn’t it be easier to have a simple, almost fool proof system.., huh.., huh..? Yeah, that’s what I thought.

The following design could have been realised in many ways but I chose the miro-controller route just because I wanted more practice(no other reason). It’s a potentially compact (the prototype is a little larger than life but what the hey, it’s a prototype) and there’s more scope for play abbility, I can change the software to get new functions while keeping the same design. And I’ll tell you something for nothing, it’s simple but it does what it says on the can.

Router Reset In Action

Specification

Voltage input:

6V – 24V dc per channel

Output Voltage:

Same as channel input

Max current output:

1A per channel

Circuit Diagram

Parts List

U1:

LM7805 voltage regulator

U2:

PIC12F675

D1 – D4:

1N4001 rectifier diodes

D5:

3mm LED

C1, C2:

100nF capacitor (rated atleast over 12V)

R1:

1K

R2:

270R

R3, R4:

1K

RLY1, RLY2:

SPDT 6V relay

Q1, Q2:

NP transistor

SW1:

SPST non latching

Explaination

Power from ‘Modem PSU’ is passed on to U1 which along side C1, C2, D1 and D2 form a simple 5V regulator for the circuit. The power comes from the actual power supply that comes with your modem which means we don’t need a seperate transformer to power the circuit (although this could be a possibility). C1 and C2 are decoupling capacitors which help smooth out instabilities in the power supply. D1 prevents U1 from being damaged by negative voltages while D2 protects the delicate circuitry in U1 from potential discharges from C2 and any other part of the ciruit.

R1 holds the reset pin of U2 (MCLR) high ensuring that it only resets when SW1 is pressed. When SW1 is pressed, the MCLR pin goes low which resets U2 and starts the power up sequence.

R2 is a current limiting resistor for D5 giving approx 20mA.

R3 and Q1 form a transistor switch for RLY1. D3 prevents any stray back emf from damaging the circuit as a whole. (R4, Q1 and D4 are the same as above but for switching RLY2).

RLY1 and RLY2 switch the voltage from ‘Modem PSU’ and ‘Router PSU’ to the output terminal/connector. These are wired in the N/C (normally closed) configuration to help preserve power (when relay is activated the power to the terminal is removed and visa versa.

U1 is the heart of the circuit. It is a PIC12f675 micro-controller which performs all the timing required for this project.