Posted September 16, 2016
low rated
I'm attempting to make an alarm for a freezer so that if the door gets left open, after 1 minute or so, an alarm will sound.
I've got something similar to the schematic below. When the switch is open the capacitor starts to discharge through the base of the transistor but I have the LED in parallel with the transistor so that when the capacitor is discharged, the LED turns on. This is working fine, however I cant make the delay long enough. If I increase the capacitor value or the transistors base resistor, the delay time is longer, however because the capacitor is discharging slower the LED/Alarm is gradually faded on which I don't really want. I would like the alarm/LED to come on as suddenly as possible.
Is there a way for me to increase the delay but keep the alarm turning on relatively suddenly?
As a footnote, I do not want to use any ICs (i.e. the 555 timer http://www.kynix.com/Search/NE555.html)
someone answered me, is this right?
To get a sharper turn on for the LED you need to increase the gain of the circuit. For those using ICs a comparator circuit would be used to compare the capacitor voltage to a reference level. Once the threshold was crossed the very high gain of the comparator would cause the output to quickly change and light your alarm LED.
Since you want to stay with simpler descrete components the next simplest approach for you to increase the gain of your circuit would be to connect two NPN transistors up in a darlington configuration. Darlington circuits will not fully saturate the output transistor and so you would have to adjust the resistor in series with the LED to achieve the same LED brightness.
To get a sharper turn on for the LED you need to increase the gain of the circuit. For those using ICs a comparator circuit would be used to compare the capacitor voltage to a reference level. Once the threshold was crossed the very high gain of the comparator would cause the output to quickly change and light your alarm LED.
I've got something similar to the schematic below. When the switch is open the capacitor starts to discharge through the base of the transistor but I have the LED in parallel with the transistor so that when the capacitor is discharged, the LED turns on. This is working fine, however I cant make the delay long enough. If I increase the capacitor value or the transistors base resistor, the delay time is longer, however because the capacitor is discharging slower the LED/Alarm is gradually faded on which I don't really want. I would like the alarm/LED to come on as suddenly as possible.
Is there a way for me to increase the delay but keep the alarm turning on relatively suddenly?
As a footnote, I do not want to use any ICs (i.e. the 555 timer http://www.kynix.com/Search/NE555.html)
someone answered me, is this right?
To get a sharper turn on for the LED you need to increase the gain of the circuit. For those using ICs a comparator circuit would be used to compare the capacitor voltage to a reference level. Once the threshold was crossed the very high gain of the comparator would cause the output to quickly change and light your alarm LED.
Since you want to stay with simpler descrete components the next simplest approach for you to increase the gain of your circuit would be to connect two NPN transistors up in a darlington configuration. Darlington circuits will not fully saturate the output transistor and so you would have to adjust the resistor in series with the LED to achieve the same LED brightness.
To get a sharper turn on for the LED you need to increase the gain of the circuit. For those using ICs a comparator circuit would be used to compare the capacitor voltage to a reference level. Once the threshold was crossed the very high gain of the comparator would cause the output to quickly change and light your alarm LED.
Post edited September 20, 2016 by krystal940
