03-05-2015, 05:19 PM
Thanks for reading my post Tony, and for looking at the pics and for your comments.
As I understand the function of the circuit, until the bell push is pressed, pin 2 is held high and there's no output at all on pin 3, but when the button is pressed, that takes pin 2 low and pin 3 goes high for a period of time determined by the combination of R1/C1 on pins 6+7. The voltage at the output of pin 3 is near enough the rail voltage, and can supply enough current (up to 200mA I believe) to power the relay coil. The relay is a two pole changeover type and I've used the normally open contacts when not energised, so when pin 3 of the IC energises the coil, those contacts close, and that allows current to flow to the bell from the bell transformer.
The problem I have is that as soon as power is applied to the circuit, pin 3 goes high and the relay pulls in, which it ought not to do until the bell push is pressed.
Re the PCB layout Tony, yes, I've shown the track side as it appears from beneath, so it's a mirror image of the component side of the PCB. To clarify the layout, I've annotate the track layout to show pins 1 & 8, so as will be seen pins 4 & 8 are joined and go to the + rail, and pins 6 & 7 are joined and go to the pre-set pot and the 100uF cap which set the timer duration. I've also shown the relay contacts and as you'll see, what I've done is to simply wire the two poles in parallel, so each pair of relay contacts share the current through them in the bell circuit from the low voltage AC which powers the bell.
I hope that clarifies what I've done.
Nothing quite leaps off the page at me - I guess I must be having an intellectual interlude (AKA 'senior moment')!
I've crawled over the circuit with a magnifying glass, and if only out of curiosity, I've added a .01uF cap on pin 5 as shown in the alternative circuit, to no avail.
I know that 555s have been around for decades so this circuit will be very commonplace and well tried, so it's not as though I'm pushing the frontiers of science.
As I understand the function of the circuit, until the bell push is pressed, pin 2 is held high and there's no output at all on pin 3, but when the button is pressed, that takes pin 2 low and pin 3 goes high for a period of time determined by the combination of R1/C1 on pins 6+7. The voltage at the output of pin 3 is near enough the rail voltage, and can supply enough current (up to 200mA I believe) to power the relay coil. The relay is a two pole changeover type and I've used the normally open contacts when not energised, so when pin 3 of the IC energises the coil, those contacts close, and that allows current to flow to the bell from the bell transformer.
The problem I have is that as soon as power is applied to the circuit, pin 3 goes high and the relay pulls in, which it ought not to do until the bell push is pressed.
Re the PCB layout Tony, yes, I've shown the track side as it appears from beneath, so it's a mirror image of the component side of the PCB. To clarify the layout, I've annotate the track layout to show pins 1 & 8, so as will be seen pins 4 & 8 are joined and go to the + rail, and pins 6 & 7 are joined and go to the pre-set pot and the 100uF cap which set the timer duration. I've also shown the relay contacts and as you'll see, what I've done is to simply wire the two poles in parallel, so each pair of relay contacts share the current through them in the bell circuit from the low voltage AC which powers the bell.
I hope that clarifies what I've done.
Nothing quite leaps off the page at me - I guess I must be having an intellectual interlude (AKA 'senior moment')!
I've crawled over the circuit with a magnifying glass, and if only out of curiosity, I've added a .01uF cap on pin 5 as shown in the alternative circuit, to no avail.
I know that 555s have been around for decades so this circuit will be very commonplace and well tried, so it's not as though I'm pushing the frontiers of science.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
