I also just tested it and it works fine for me. I am using a 2.4V NiCd battery, a red and green LED, a 74HC240 chip and 1uf tantalum timing caps. The solar cell is a BP3733.
It's been a while since I played with this circuit so I did some tests to verify my assumptions how it would behave with and without the positive feedback resistor.
Without the feedback resistor , the pummer LEDs sputter on and off when the solar cell is slowly covered up simulating dusk. This is because the circuit oscillates rapidly when the input voltage at pin 1 and 19 is near the threshold. With the 100K feedback resistor, this wasteful oscillation is almost eliminated and I changed the positive feedback resistor from 100K to 51K the circuit switches cleanly on and off.
This positive feedback is simple to explain:
When the voltage at pin 1 and 19 is high during charging, all 240 output pins are tri-stated (internally disconnected), the pummer is off and the 51K feedback resistor is isolated on pin 18 and has no effect on the circuit. As the light fades and the solar cell voltage drops, the input voltage at pin 1 and pin 19 reaches the turn-on threshold which causes output pin 18 to turn on and swith to grounded potential. With one side of the 51K resistor connected to ground through pin 18 and the other connected to the solar cell (+) terminal, this switches the 51K resistor in parallel with the 100K load resistor across the solar cell. The extra loading drops the solar cell voltage even more and causes the circuit to turn on harder. That is why it is is called positive feedback.
The circuit is works every time so the design is reliable. There is a small improvement to come but I will explain that after you have the basic circuit working.
For now disconnect the solar cell (+) and battery.
Connect pin 1/19 directly to ground. Reconnect the battery between pin 10/20. Is the pummer working? If not, the pummer connections (pin 3-9 and pin11-17) or the LEDs/1000uf cap connections have a problem. Double check. If it is working, the solar cell, diode, 100K load resistor pr 51K feedback resistor connections are suspect.
Subject: Re: [beam] Re: 74ac240 PUMMER, need your help!
I just breadboarded this circuit, and can't get it to work properly. I'm pretty sure I have all the connections set up correctly, and I'm using a 3.6V NMHi battery. Yes, I'm covering up the solar cell to test dark conditions :)
I get a brief flash when connecting the battery, so it's not totally dead.
Does it matter if the 1uF caps are monolithic or electrolytic? I was testing with the former.
Note the positive feedback resistor from pin 18 to pin 1/19 which adds an extra load on the solar cell when the circuit turns on. This causes the circuit to efficiently snap on and off.
The 1000uF cap should be rated 10V as it is used in a non-polarized mode with small forward and reverse voltage pulses applied.
The 1000uF cap acts as a charge pump which doubles the available output voltage to drive the LEDs even when the battery voltage drops below the turn on voltage of the LEDs.
The two LEDs turn on brightly and nicely fade (pum) after each flash. The 10K across the LEDs can be increased to make the fade out longer but if removed completely, the brightness will be less.
I recently built this pummer, you can see a video of it here http://www.youtube.com/watch?v=l76V9x26qRE . I need your help in solving the mystery of the LED!...
I recently built this pummer, you can see a video of it here http://www.youtube.com/watch?v=l76V9x26qRE . I need your help in solving the mystery of the LED!...
HI Brian, Try the following: 1. Reduce from two batteries to one battery 2. Switch the polarity of LED If you have a MultiMeter, measure the output from...
First, brilliant idea to demonstrate the problem on youtube. Second, the problem is most likely due to floating connections on the 240 chip Vcc and/or...
... Pin 20 to +V of the battery? Tristate enable is the function of pin 1 and pin 19. Can be best described as: the output pins of the chip are isolated and...
... wilf, I have just discovered something with my pummer, when i short circuit it between pins 17 and the capacitors 1000uf cap, which is also connected to...
Hi Brian, First off, what pummer circuit did you use? Is it similar to this one --> http://download.yousendit.com/99DE84A533226C7E I got this schematic from...
Check all your solder joints and connections. This is probably the single most significant cause for why my bots don't work when they should. Also, the small...
The faint LED glow is probably due to pins 5/7/9 being low and the LED current supplied by the 1K resistor. That would indicate possibly a missing connection...
... LED current supplied by the 1K resistor. ... 12/14/16 and pin 18. ... which I take it is the paperclip frame. Normally the frame would be connected to...
... resistor ... + ... exactly ... best ... is "high" ... single ... versions of ... the chip ... solder ... short ... is ... area,. ... video, ... and ... i...
I will post it in the morning. wilf ... From: ads1977 To: beam@yahoogroups.com Sent: Thursday, November 23, 2006 2:14 AM Subject: [beam] Re: 74ac240 PUMMER,...
Better late that never, here is the "improved" pummer circuit. http://www.user.dccnet.com/wrigter/pum.gif Note the positive feedback resistor from pin 18 to...
I just breadboarded this circuit, and can't get it to work properly. I'm pretty sure I have all the connections set up correctly, and I'm using a 3.6V NMHi...
Thanks, thats awesome.. I'm going to try out that circuit as soon as I've got some time to wire it up. ... adds an extra load on the solar cell when the...
I also just tested it and it works fine for me. I am using a 2.4V NiCd battery, a red and green LED, a 74HC240 chip and 1uf tantalum timing caps. The solar...
Oops, one small error on the schematic. Remove the jumper between pin 14 and pin 16. It will still work with this jumper in but the timing of the two LED...
The schematic also shows a AC240, rather than an HC240 as you mention you used. I'll try the corrected circuit with an 74HC240 tomorrow. Simon -- New robot...
Hello Simon, This changes everything. I am simply awed by your remarkable background and your website. It will no doubt inspire some heady discussion here on...
... Thank you for the compliments! I hope to be building many more robots, and, eventually, developing my own circuits. ... I just breadboarded the circuit...
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