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| [SOLVED] NE555 as a tone generator for Morse code practice |
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| StillTrying:
--- Quote from: Larzzon on September 15, 2019, 07:28:50 pm ---although i think i blew my speaker (cause i did not have any resistor before the speaker) so regardless i need to go grab some spares tomorrow. --- End quote --- The speaker would only be blown if you were using quite a powerful PSU. It's quite safe to test a 8R speaker by connecting it to a small 1.5V battery, there should be quite a noticeable click. |
| Zero999:
It's possible you blew the speaker, but I think it's more likely you blew the IC. The maximum output current rating of the 555 is only 200mA and an 8 Ohm speaker will draw more than that, so a current limiting resistor, in the form of an extra resistor in series with the volume control, is necessary. This will ensure the current won't be too high, even when the potentiometer is set to the lowest value. The 555's output from the 555 will drop a couple of volts, so a 10R resistor will probably do. I doubt a 1k potentiometer in series would affect the volume much with a piezo transducer. The potentiometer should be connected as a potential divider, i.e. the wiper should be connected to the piezo and one end connected to pin 3 and the other to 0V. |
| soldar:
I have assembled this type of circuit many times. The output of pin 3 is more than enough to drive a small conventional speaker and I generally need to insert a resistor in series in order to lower the volume and protect the speaker. By switching the power supply the first few cycles are unstable until the frequency stabilizes so I generally do not switch the power supply but leave it running continually and switch the speaker on/off. That way the problem is avoided. |
| Zero999:
soldar, What power supply voltage were you using? Unless the capacitor is small enough to limit the current, running this circuit at 9V with an 8 Ohm speaker, is marginal design. Assuming the 555 output stage drops 2.5V, the peak-to-peak voltage will be 9-5V = 4V, giving an RMS voltage of 2V, which is a current of 250mA with an 8Ohm speaker, when the IC is only rated to 200mA maximum. You'll probably get away with it, at low duty cycles or because the battery voltage might drop a bit, but it's not recommended. I admit I was being conservative by suggesting 10R. A lower value can be used if it's not loud enough, but I think the volume will be ample. |
| MarkF:
--- Quote from: soldar on September 15, 2019, 09:06:31 pm ---By switching the power supply the first few cycles are unstable until the frequency stabilizes so I generally do not switch the power supply but leave it running continually and switch the speaker on/off. That way the problem is avoided. --- End quote --- Switching the RESET pin would probably be a better option than switching either the power pin or the speaker. |
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