Hi folks,
First up, I'm sure there is a really simple answer to this, and back in my younger years I'm sure I'd think this question is a newbie one, at best. But at 40, and not having kept up my tinkering, I have lost most of the knowledge I once possessed, so I turn to you all for help, and I'm thankful forums like this exist.
OK, to the circuit: I purchased a "Flexitimer Kit" from Jaycar which I have put together and it works wonderfully as they designed it. It uses a 555 oscillator, going to a 4020 ripple (binary) counter. When the circuit is powered up, the 555 starts cycling, pulsing the 4020, and when the selected (with a jumper) pin on the 4020 eventually goes high (12v or so), it triggers (through a 39k resistor) a BC338 transistor which activates a relay. When the transistor goes high, it stops the 555 from counting any further by pulling pin 4 high as well. All perfect as they designed it. (Although I also added a reset switch to pin 11 of the 4020 to start the cycle again, rather than just working on power up.)
Now, the problem: The problem is that I need it to be a countDOWN timer. Meaning, what I want to happen is the relay is to switch ON during the counting up period, and once it gets to the selected time, the relay should go off. So what I figure is that I need the transistor to be switched on when the selected output pin on the 4020 is low (zero volts) and switched OFF when the 4020 goes high.
In other words, I want (I think) the transistor to work in reverse, but do you think I can think of any way to make it do that? Is there any such thing? Do I need a "not" gate in front of the transistor, and if so, can I make one using only a couple of components? (I don't want to add another IC to make this work.)
I hope I have been clear enough in my description of the circuit. I know it must be hard to envisage these things without a circuit diagram in front of you, but I can scan it in for people to see if needed. But in summary, all I really need is a better understanding of transistors. ;-)
Thanks in advance for all your help.
Mark
First up, I'm sure there is a really simple answer to this, and back in my younger years I'm sure I'd think this question is a newbie one, at best. But at 40, and not having kept up my tinkering, I have lost most of the knowledge I once possessed, so I turn to you all for help, and I'm thankful forums like this exist.
OK, to the circuit: I purchased a "Flexitimer Kit" from Jaycar which I have put together and it works wonderfully as they designed it. It uses a 555 oscillator, going to a 4020 ripple (binary) counter. When the circuit is powered up, the 555 starts cycling, pulsing the 4020, and when the selected (with a jumper) pin on the 4020 eventually goes high (12v or so), it triggers (through a 39k resistor) a BC338 transistor which activates a relay. When the transistor goes high, it stops the 555 from counting any further by pulling pin 4 high as well. All perfect as they designed it. (Although I also added a reset switch to pin 11 of the 4020 to start the cycle again, rather than just working on power up.)
Now, the problem: The problem is that I need it to be a countDOWN timer. Meaning, what I want to happen is the relay is to switch ON during the counting up period, and once it gets to the selected time, the relay should go off. So what I figure is that I need the transistor to be switched on when the selected output pin on the 4020 is low (zero volts) and switched OFF when the 4020 goes high.
In other words, I want (I think) the transistor to work in reverse, but do you think I can think of any way to make it do that? Is there any such thing? Do I need a "not" gate in front of the transistor, and if so, can I make one using only a couple of components? (I don't want to add another IC to make this work.)
I hope I have been clear enough in my description of the circuit. I know it must be hard to envisage these things without a circuit diagram in front of you, but I can scan it in for people to see if needed. But in summary, all I really need is a better understanding of transistors. ;-)
Thanks in advance for all your help.
Mark