Relay unit for operation from MRT computer program
Block schematic diagram
Parts list
C1 |
1 CAP,470 uF 63 Volt |
D1,D2 |
2 Diodes, 1N4007 |
F1 |
1 FUSE, 400mA |
J1 |
D25-connector, pin 7 |
J2 |
D25-connector, pin 4 |
D3 |
Red LED |
Q1 |
1 N-channel Power MOSFET , IRF20 |
PL1 |
IEC320, mains input, 240VAC |
PL2 |
240 VAC switched outlet to bulb. 60W maximum |
R4 |
100K 0.25W |
R5 |
6K8 0.25W |
R6 |
470 0.5W |
K1 |
RELAY, 12V coil, DPDT contacts rated at 250 VAC, 10A |
R1 |
1K8 2W |
S1 |
Mains switch, with neon lamp indicator and built-in fuse holder |
T1 |
240V input 18V output, 6VA |
U1 |
12V regulator TS7812CZ |
C3,C3 |
0.01uF, 1000V |
R7,R8 |
R33 0.25W |
This unit is designed to be connected to the 25 pin D-connector used for serial I/O on PCs. The unit is triggered by RS232 -+12V logic levels on pin 4 (RTS) and is designed to switch a resistive 240VAC 25W load such as a lamp. It is to be used together with a computer program that can toggle the RS232 RTS line as required.
Summary of circuit function.
T1A, C1, D1, R1, U1 comprise a simple regulated 12Volt DC power supply to provide current to actuate relay K1 when Q1 is switched on.
The -+ 12V inverted logic on pin 4 is fed via protective resistor R5 to the gate of the N-channel Power MOSFET Q1. Q1 gate is tied to the common rail via R4 to ensure that Q1 will switch off under input disconnection conditions.
When Q1 switches on relay K1 operates and switches the 240 VAC supply on to PL2. The indicator LED is across the relay coil and lights to provide an ‘on’ indicator and as a warning that mains power exists at PL2.
C2, C3, R7, R8 are surge suppression components. D2 protects Q1 from voltage spikes generated in K1 during switching.
Usage:
Connect the RS232 input to the serial port of PC using any required adapters and cables.
Connect 240VAC mains supply to mains input socket PL1
Connect a 240 V mains bulb to mains output socket PL2
Start the MRT program and ensure that the program option to output to serial port is switched on.
Use the MRT program to play Morse code.
The lamp should flash as code is played.
Notes:
The RS232 inputs are isolated from the unit case and ground to avoid mains surge currents during switching from damaging the PC
Switching input is expected on pin 4 of the 25 pin connector. This is defined as Request to Send (RTS) by the RS232 standard. Some cables and adapters can exchange pins 4&5. If this occurs then the software should be set to toggle pin, 5 Clear to Send (CTS), instead.
Pin 7 is serial port logic common. Pin 7 must not be connected to the relay unit case or mains ground.
Absolute maximum rating for RS232 inputs to this unit: -+ 20V
An earth connection MUST be provided to this unit via the mains input connector.
A 400mA fuse is fitted to the live (L) side of the mains input and is accessed via a small tray built-in to the mains input socket. There is also a spare fuse in this tray.
Maximum bulb rating is 60W. Recommended rating is 25W. The bulb must be an incandescent type. Fluorescent or energy saving bulbs must not be used.
The relay contacts are rated at 10A 250VAC. Inrush currents to a cold incandescent bulb can be as much as twelve (12) x hot current for up to 50ms. A 60W bulb can take up to 3A each time it is switched on. This means that lamps that are flashing at low rates will have a shorter life than normal.
Nominal relay switching speed is 20ms. This limits the maximum code speed, but it is likely that the thermal inertia of the bulb will impose a lower limit.